Some of the reasons could be:
Moody was a very powerful auror and it would have been very difficult
to keep him in control for that long. Barty Crouch Jr. was able to overpower him by taking him by surprise, and having help from Wormtail. Even then, he only just managed to do it.
Wormtail and I did it. We had prepared the Polyjuice Potion
Probably not - it seemed to be just the effect of being in Azkaban.
It seems most likely that it was the effect of being in Azkaban. Sirius told Harry that Barty Crouch Jr. died in Azkaban, and that many others did as well. He also said Barty Crouch Jr. looked sickly when he arrived in Azkaban, but whether that was due to a separate illness is never ...
It was a plausible reason for Moody to turn Draco into a ferret.
Barty Crouch Jr. didn’t necessarily actually care that Draco attacked Harry when Harry’s back was turned. (It’s possible, though, that he did, but either considered an opponent with their back turned different to being outnumbered, or was simply a bit hypocritical.) However, it would certainly ...
He does deny it.
"Father, I didn't!" shrieked the boy in chains below. "I didn't, I swear it, Father, don't send me back to the Dementors -"
"Mother!" screamed the boy below, and the wispy little witch beside Crouch began to sob, rocking backwards and forwards. "Mother, stop him, Mother, I didn't do it, it wasn't me!"
(Goblet of Fire, ...
Moody did fight, but Barty Crouch Jr. and Wormtail subdued him.
Barty Crouch Jr. describes how he and Wormtail captured Moody while being questioned under Veritaserum. He says that Moody struggled, and there was a commotion, but they did still manage to subdue him. From this, it sounds like it was difficult to capture Moody, though they were able to.
I'm not entirely convinced that was what Crouch hated them for; consider what he says to Harry near the end (emphasis mine):
"I asked you," said Moody quietly, "whether [Voldemort] forgave the scum who never even went to look for him. Those treacherous cowards who wouldn't even brave Azkaban for him. The faithless, worthless bits of filth who were brave ...
Yes he most likely did, but he denied it to save himself from Azkaban.
The Death Eaters that tortured the Longbottoms were Bellatrix, Rabastan and Rodolphus Lestrange, and Barty Crouch Jr. However, except for Bellatrix, most of them deny their involvement. It certainly seems, though, like the four Death Eaters were all caught in the act of torturing the ...
He was very obviously suspicious
I'm really not sure what else needs to be said here:
After Moody licks his lips, we can clearly see Crouch's facial expression turn from polite confusion to startled horror and deep, deep discomfort. Very shortly afterwards, in one of Dumbledore's memories, we see that this was a nervous ...
Barty Crouch was pretending to be Alastor Moody at this point in time. He's trying to act in such a way that people won't suspect that he isn't who he says he is.
This is why he teaches the students actual, applicable, skills, such as how to throw off the Imperious Curse. Him acting in a way that matches Moody's honorable character is part of that.
How did he do the things he did?
Barty Jr. was a highly accomplished wizard.
His father states that he (Crouch Jr) earned passing grades in every subject taught at Hogwarts, something only accomplished by a very few high status wizards;
‘Yes, my son has recently gained twelve O.W.Ls, most satisfactory,
yes, thank you, yes, very proud indeed. Now, if ...
The Imperius curse, like all magic in Harry Potter, isn’t foolproof. As Crouch (impersonating Moody) himself explains it in class:
‘The Imperius curse can be fought, and I’ll be teaching you how, but it takes real strength of character, and not everyone’s got it. (…)’
(Goblet of Fire, chapter The Unforgivable Curses)
It is difficult to place a ...
First of all, Barty Crouch Sr. was not at the Second Task in the book.
In the book, Barty Crouch Sr. was described as being "too ill" (really it was that he was beginning to fight the Imperius Curse placed on him and was harder to control) to fulfill his role as judge for the Second Task. In the book, Percy Weasley, his personal assistant (as well as Ron's ...
There is no evidence in canon, that the killing of animals can split your soul. The splitting of the soul is caused by murder, which according to Merriam Webster is
the crime of unlawfully killing a person especially with malice
Merriam Webster - Murder
Barty Crouch Jr. isn't killing a person, and according to the Halfblood Prince, the curse itself ...
Being in Azkaban is a terminal illness.
In spite of opposition from many wizards, among them experts on both Dementors and buildings with Azkaban’s kind of Dark history, Rowle carried out his plan and soon a steady trickle of prisoners had been placed there. None ever emerged. If they were not mad and dangerous before being placed in Azkaban, they swiftly ...
Barty Crouch Jr. has more control being Moody than Imperiusing the real one.
First of all, and perhaps most importantly, Barty Crouch Jr. would be able to strategically plan his actions much better when doing them himself than having to puppet Moody through them. There's a definite benefit in being able to take over the role of Moody, and do everything ...
No, Barty Crouch Junior wasn't dying.
There's no indication that Barty Crouch Junior was dying at all.
The reason the Dementors viewed Crouch as a "dying person" was because of the Polyjuice Potion.
"They came to visit me. They gave me a draught of the Polyjuice Potion, containing one of my mother's hairs. She took a draught of Polyjuice Potion, ...
First off, it would be in character for Moody to respond like this.
‘I don’t like people who attack when their opponent’s back’s turned,’ growled Moody, as the ferret bounced higher and higher, squealing in pain. ‘Stinking, cowardly, scummy thing to do …’.
Goblet of Fire - Chapter 13: Mad-Eye Moody
Secondly, four Death Eaters vs two Aurors is a bit ...
Barty Crouch Jr.'s experiences changed his outlook on life.
Consider that he'd been in captivity for what could have been a long time, and guarded by Dementors. In such an atmosphere, even the most loyal of loyalists might be driven by terror and panic to deny what they believed in, clinging to the vain hope that they might be able to escape.
Here's the ...
Most likely, yes
Worth remarking that Dean brings this up to Umbridge in Order of the Phoenix:
"It is my understanding that my predecessor not only performed illegal curses in front of you, he actually performed them on you —"
"Well, he turned out to be a maniac, didn't he?" said Dean Thomas hotly.
Order of the Phoenix Chapter 12: "Professor ...
It almost certainly didn’t.
It’s unlikely killing insects would affect someone’s soul. Typically, killing spiders is treated very different than killing humans. Slughorn explains that it’s caused by murder specifically, not just killing.
“Well,’ said Slughorn uncomfortably, ‘you must understand that the soul is supposed to remain intact and whole. ...
Because Barty Crouch Jr. is the type of person who will harshly judge others, but not himself.
While you could argue, he objected to Malfoy attacking Harry from behind as a cover for his disguise as Moody, I find this argument flimsy because he also immediately punished Malfoy by turning him into a ferret.
We know little of Moody's actual character, as ...
His father is so obsessed with his work in the Ministry that it's like his work is more significant than his family. But then, you asked the specific reason of why was Barty Crouch Jr. a Death Eater?
There is no specific reason based on what I read in book four.
It all just leads to a fake Mad-Eye who wants the Dark Lord to be pleased with him after what ...
The jinx didn't affect Barty Crouch Jr
Alastor Moody took the job of DADA teacher before the 1994 school year began, after being convinced out of retirement by Dumbledore. However, before he could start teaching, he was attacked at his own home by Crouch Jr. From Chapter 35 of Harry Potter and the Goblet of Fire, during Crouch Jr's confession, we learn all ...
We only have conjecture to base this on, but in the books, it is not implied that Barry crouch knew anything.
In the movie, there is a scene which implies that he suspects something is up; Barty Jr has a twitch of sorts -- poking his tongue out like a snake. Barty Sr might have known of this quirk, as shown in the pensive scene, Barty Jr performs it in ...
It is unclear
It is not stated anywhere in the books that Legilimency was applied to fake Moody/Crouch jr. The conclusion Dumbledore made (quoted in the question) is based on pure logic, not Legilimency or any other magical means.
So unless any other canon information is available, we cannot tell.
The answers so far have missed out something. Credit. If it was Barty Crouch Jr. doing all the deeds in order to bring Voldemort's sinister plots to fruition, he could claim that Voldemort was indebted to him. There wouldn't be anybody else in the direct link between Crouch and Harry being given to Voldemort.
As you can see from this quote, credit was very ...
Without confirmation from the screenwriters, we can only speculate why they changed this in the movie. But I imagine because Crouch Sr. wasn't portrayed as the same power-grasping man.
In fact, we don't really see much of him in the movie, so it could be because it would require too much time and exposition to fit in this scene accurately. So they instead ...
Dumbledore and Snape didn't just read people's minds all the time.
While they could use Legilimency to find out what everyone is thinking all the time, they don't just go around reading everyone's mind without any reasons to do it. Remember, they're not evil, and they'd certainly be aware that Legilimency is an invasion of privacy. It's unlikely that they ...
Even Mad-Eye Moody's famed paranoia has limits - apparently rodent proofing his house exceeded them.
From an entirely unrelated series:
“You want to send in harnessed ferrets through a laundry vent?” Augustine clearly had difficulty coming to terms with that idea.
“Yes,” Cornelius said.
I blinked. “Wouldn’t the vent be secured by an alarm?”