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.
- Anakin drew upon the Dark Side when he killed the Tusken Raiders (long before becoming a Sith apprentice).
- Anakin doesn't try to repent after that, instead justifying his actions to himself.
- 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.