When did Anakin fall to the dark side?

Question

If giving in to your anger and hatred when drawing on the Force is considered to be the act that seals your fall to the dark side, and that its temptation, once tasted, prevents you from turning back to the light without intense difficulty, then has Anakin actually drawn on the dark side before in the decade plus before he became Palpatine's apprentice?

If he had already indulged in the dark side prior to his official fall, then has he already somehow fallen long before, and the Jedi merely didn't sense it? Or did he manage, with or without assistance, to subconsciously claw his way back to the light after every time, even though it's supposed to be near impossible? Or did he simply stay in the light because he didn't realise it?

Answer 1

It's hard to say, because he was sliding into dark side as a gradual process.

But the notable turning points were:

• His slaughter of Tuskens (Quotes from AotC novelization)

  At that time, the only meaning, the only purpose, that Anakin could fathom was that of the rage building within him, an anger at losing someone he did not wish to give up.
  Some small part of him warned him not to give in to that anger, warned him that such emotions were of the dark side.
  Then he looked at Shmi lying there, so still, seeming at peace but covered with the clear evidence of all the pain that had been inflicted upon her poor body these last days.
  The Jedi Padawan climbed to his feet and took up his lightsaber, then boldly strode through the door.

  and later, when he recounts it to Padme:

  Why couldn’t I save her?” Anakin asked. “I know I could have!”
  “Annie, you tried.” She squeezed him a bit tighter. “Sometimes there are things no one can fix. You’re not all-powerful.”
  He stiffened at her words and pulled away from her suddenly—and angrily, she realized. “But I should be!” he growled, and then he looked at her, his face a mask of grim determination. “And someday I will be!”
  “Anakin, don’t say such things,” Padmé replied fearfully, but he didn’t even seem to hear her.
  “I’ll be the most powerful Jedi ever!” he railed on. “I promise you! I will even learn to stop people from dying!”
  ... He ignored her. “Not just the men,” he went on. “And the men are the only fighters among the Tuskens. No, not just them. The women and the children, too.” His face contorted, as if he was teetering between anger and guilt. “They’re like animals!” he said suddenly. “And I slaughtered them like animals! I hate them!”
  ...
  “Why do I hate them?” Anakin asked her.
  “Do you hate them, or do you hate what they did to your mother?”
  “I hate them!” he insisted.
  “And they earned your anger, Anakin.”
  He looked up at her, his eyes wet with tears. “But it was more than that,” he started to say, and then he shook his head and buried his face against the softness of her breast.
  A moment later, he looked back up, his expression showing that he was determined to explain. “I didn’t … I couldn’t …” He held one hand up outstretched, then clenched it into a fist. “I couldn’t control myself,” he admitted. “I … I don’t want to hate them—I know that there is no place for hatred. But I just can’t forgive them!”
  “To be angry is to be human,” Padmé assured him.
  “To control your anger is to be a Jedi,” Anakin was quick to reply, and he pulled away from her and stood up, turning to face the open door and the desert beyond.

• His duel with Dooku:

  He {{Dooku}} understood how Skywalker was getting stronger. Why he no longer spoke. How he had become a machine of battle. He understood why Sidious had been so interested in him for so long.
  Skywalker was a natural.
  There was a thermonuclear furnace where his heart should be, and it was burning through the firewalls of his Jedi training. He held the Force in the clench of a white-hot fist. He was half Sith already, and he didn’t even know it.
  This boy had the gift of fury.
  And even now, he was holding himself back; even now, as he landed at Dooku’s flank and rained blows upon the Sith Lord’s defenses, even as he drove Dooku backward step after step, Dooku could feel how Skywalker kept his fury banked behind walls of will: walls that were hardened by some uncontrollable dread.
  Dread, Dooku surmised, of himself. Of what might happen if he should ever allow that furnace he used for a heart to go supercritical.
  (ROTS Novelization by Matthew Stover, Ch 3, Way of the Sith)

• ... and, continued from Chapter 3 to Chapter 4,

  the conclusion of that duel, when he opened up to those emotions on Sidious's command:

  “Don’t fear what you’re feeling, Anakin, use it!” he barked in Palpatine’s voice. “Call upon your fury. Focus it, and he cannot stand against you. Rage is your weapon. Strike now! Strike! Kill him!”
  ...
  == Chapter 4 ==
  This is the death of Count Dooku:
  A starburst of clarity blossoms within Anakin Skywalker’s mind, when he says to himself Oh. I get it, now and discovers that the fear within his heart can be a weapon, too.
  It is that simple, and that complex.
  And it is final.
  ...
  But for Anakin, in the fight there is only terror, and rage.
  Only he stands between death and the two men he loves best in all the world, and he can no longer afford to hold anything back. .
  That imaginary dead-star dragon tries its best to freeze away his strength, to whisper to him that Dooku has beaten him before, that Dooku has all the power of the darkness, to remind him how Dooku took his hand, how Dooku could strike down even Obi-Wan ...
  Only he stands between death and the two men he loves best in all the world, and he can no longer afford to hold anything back. That imaginary dead-star dragon tries its best to freeze away his strength, to whisper to him that Dooku has beaten him before, that Dooku has all the power of the darkness, to remind him how Dooku took his hand, how Dooku could strike down even Obi-Wan himself seemingly without effort and now Anakin is all alone and he will never be a match for any Lord of the Sith—
  But Palpatine’s words rage is your weapon have given Anakin permission to unseal the shielding around his furnace heart, and all his fears and all his doubts shrivel in its flame.
  ...
  His head has been filled with the smoke from his smothered heart for far too long; it has been the thunder that darkens his mind. On Aargonar, on Jabiim, in the Tusken camp on Tatooine, that smoke had clouded his mind, had blinded him and left him flailing in the dark, a mindless machine of slaughter; but here, now, within this ship, this microscopic cell of life in the infinite sterile desert of space, his firewalls have opened so that the terror and the rage are out there, in the fight instead of in his head, and Anakin’s mind is clear as a crystal bell.

• His murder of Dooku

  (still Chapter 4)
  “Kill him,” Palpatine says. “Kill him now.”
  In Skywalker’s eyes he sees only flames.
  ...
  “Anakin,” Palpatine says quietly. “Finish him.”
  Years of Jedi training make Anakin hesitate; he looks down upon Dooku and sees not a Lord of the Sith but a beaten, broken, cringing old man.
  “I shouldn’t—”
  But when Palpatine barks, “Do it! Now!” Anakin realizes that this isn’t actually an order. That it is, in fact, nothing more than what he’s been waiting for his whole life.
  Permission.
  And Dooku —
  As he looks up into the eyes of Anakin Skywalker for the final time, Count Dooku knows that he has been deceived not just today, but for many, many years. That he has never been the true apprentice. That he has never been the heir to the power of the Sith. He has been only a tool.
  He has existed only for this.
  ...
  To be the victim of Anakin Skywalker’s first coldblooded murder.
  First but not, he knows, the last.
  ...
  The murderer blinked again.
  Who am I?
  Was he the slave boy on a desert planet, valued for his astonishing gift with machines? Was he the legendary Podracer, the only human to survive that deadly sport? Was he the unruly, high-spirited, trouble-prone student of a great Jedi Master? The star pilot? The hero? The lover? The Jedi?
  Could he be all these things — could he be any of them — and still have done what he has done?
  He was already discovering the answer at the same time that he finally realized that he needed to ask the question.
  ...
  He’d been having a dream. He’d been flying, and fighting, and fighting again, and somehow, in the dream, he could do whatever he wanted. In the dream, whatever he did was the right thing to do simply because he wanted to do it. In the dream there were no rules, there was only power.
  And the power was his.
  Now he stood over a headless corpse that he couldn’t bear to see but he couldn’t make himself look away, and he knew it hadn’t been a dream at all, that he’d really done this, the blades were still in his hands and the ocean of wrong he’d dived into had closed over his head.
  And he was drowning.
  The dead man’s lightsaber tumbled from his loosening fingers. “I—I couldn’t stop myself …”
  And before the words left his lips he heard how hollow and obvious was the lie.
  Dooku had been murdered.
  By him.
  On purpose.
  Here in the General’s Quarters, he had looked into the eyes of a living being and coldly decided to end that life. He could have chosen the right way. He could have chosen the Jedi way.
  But instead —
  He stared down at Dooku’s severed head.
  He could never unchoose this choice. He could never take it back. As Master Windu liked to say, there is no such thing as a second chance.
  And he wasn’t even sure he wanted one.
  He couldn’t let himself think about this. Just as he didn’t let himself think about the dead on Tatooine. He put his hand to his eyes, trying to rub away the memory. “You promised we would never talk about that again.”
  “And we won’t. Just as we need never speak of what has happened here today.” It was as though the shadow itself spoke kindly. “I have always kept your secrets, have I not?”

• His choice to kill Mace Windu

  “I need him alive!” Skywalker shouted. “I need him to save Padmé!”
  Mace thought blankly, Why? And moved his lightsaber toward the fallen Chancellor.
  Before he could follow through on his stroke, a sudden arc of blue plasma sheared through his wrist and his hand tumbled away with his lightsaber still in it and Palpatine roared back to his feet and lightning speared from the Sith Lord’s hands and without his blade to catch it, the power of Palpatine’s hate struck him full-on. ...

• And, finally, Anakin's decision to become full on Sith apprentice

  “You must learn to cast off the petty restraints that the Jedi have tried to place upon your power,” he said. “Anakin, it’s time. I need you to help me restore order to the galaxy.”
  Anakin didn’t respond.
  Sidious said, “Join me. Pledge yourself to the Sith. Become my apprentice.”
  A wave of tingling started at the base of Anakin’s skull and spread over his whole body in a slow-motion shock wave.
  ...
  “It’s them or me, Anakin. Or perhaps I should put it more plainly: It’s them or Padmé.”
  ...
  “It’s just—it’s not … easy, that’s all. I have—I’ve been a Jedi for so long—”
  ...
  As the shadow beside him spoke, its words became true. From a remote, frozen distance that was at the same time more extravagantly, hotly intimate than he could have ever dreamed, Anakin handled his emotions. He dissected them. He reassembled them and pulled them apart again. He still felt them—if anything, they burned hotter than before—but they no longer had the power to cloud his mind.
  “You have found it, my boy: I can feel you there. That cold distance—that mountaintop within yourself—that is the first key to the power of the Sith.”
  ...
  On the mountain peak within himself, he weighed Padmé’s life against the Jedi Order.
  It was no contest.
  He said, “Yes.”
  “Yes to what, my boy?”
  “Yes, I want your knowledge.”
  “Good. Good!”
  “I want your power. I want the power to stop death.”
  “That power only my Master truly achieved, but together we will find it. The Force is strong with you, my boy. You can do anything.”
  “The Jedi betrayed you,” Anakin said. “The Jedi betrayed both of us.”
  “As you say. Are you ready?”
  “I am,” he said, and meant it. “I give myself to you. I pledge myself to the ways of the Sith. Take me as your apprentice. Teach me. Lead me. Be my Master.”
  Sidious raised the hood of his robe and draped it to shadow the ruin of his face.
  “Kneel before me, Anakin Skywalker.”
  Anakin dropped to one knee. He lowered his head.
  “It is your will to join your destiny forever with the Order of the Sith Lords?”
  There was no hesitation. “Yes.”
  Darth Sidious laid a pale hand on Anakin’s brow. “Then it is done. You are now one with the Order of the Dark Lords of the Sith. From this day forward, the truth of you, my apprentice, now and forevermore, will be Darth …”

Answer 2

Killing the Tusken Raiders was his first major step in a long, gradual process of falling to the Dark Side.

The Dark Side of the Force is so dangerous because it's so addictive. The tricky thing about addictions is that you don't notice the change happening.

YODA: If once you start down the dark path, forever will it dominate your destiny. Consume you, it will.

Anakin drew upon the Dark Side when he killed the Tusken Raiders. He was full of anger, hatred, and rage. That was his first major step into the Dark Side. He always had an attachment issue, but now that he'd let that attachment lead him to call on the Dark Side once, he was less likely to pull himself out of it.

After the killings on Tatooine, Anakin doesn't go through any sort of repentance process, seeking instead to excuse himself and justify his actions.

ANAKIN: They're like animals, and I slaughtered them like animals.

Anakin doesn't try to solve his attachment issue either, despite Yoda's attempts to counsel him.

YODA: Train yourself to let go of everything you fear to lose.

Instead of learning to have a "come what may" attitude, Anakin becomes even more obsessed with holding onto those he cares for. This inability to let go leads him to follow the Dark Side and kill Mace Windu and follow Sidious.

In Summary:

1. Anakin drew upon the Dark Side when he killed the Tusken Raiders (long before becoming a Sith apprentice).
2. Anakin doesn't try to repent after that, instead justifying his actions to himself.
3. Anakin doesn't try to solve his attachment issue, becoming even more paranoid about losing those he loves.

So it was essentially all downhill after the killings on Tatooine. He became a Sith apprentice after killing Mace Windu, but Anakin had been moving farther from the Jedi way and closer to the Dark Side for a while now.

Answer 3

Yoda foreshadowed Anakin's fall in The Phantom Menace.

YODA (to young ANAKIN): Fear is the path to the dark side. Fear leads to anger, anger leads to hate, hate leads to suffering.

In Attack of the Clones, Anakin's fear for his mother's safety led him to Tatooine and to the Tusken Raiders and to wholesale slaughter of everyone at their encampment. Not only did Anakin kill in a vengeful rage, he relished the bloodshed. This was Anakin's irreversible start down the path of the dark side of Force.

Answer 4

I think the very beginning of the process started in Episode II. Not very much though, the true formation took place in Episode III. But I believe he turned into Darth Vader in Episode III after he assisted in the death of Mace Windu.

Answer 5

Losing his Padawan, Ahsoka Tano, also helped.