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 scene from the book (emphasis mine):
The Dementors placed each of the four people in the four chairs
with chained arms that now stood on the dungeon floor. There was
a thickset man who stared blankly up at Crouch; a thinner and
more nervous-looking man, whose eyes were darting around the crowd; a woman with thick, shining dark hair and heavily hooded
eyes, who was sitting in the chained chair as though it were a throne; and a boy in his late teens, who looked nothing short of petrified. He was shivering, his straw-colored hair all over his face,
his freckled skin milk-white. The wispy little witch beside Crouch
began to rock backward and forward in her seat, whimpering into
Crouch stood up. He looked down upon the four in front of him, and there was pure hatred in his face.
“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 —”
“No! Mother, no! I didn’t do it, I didn’t do it, I didn’t know! Don’t send me there, don’t let him!”
The Dementors were gliding back into the room. The boys’ three companions rose quietly from their seats; the woman with the heavy-lidded eyes looked up at Crouch and called, “The Dark Lord will rise again, Crouch! Throw us into Azkaban; we will wait! He will rise again and will come for us, he will reward us beyond any of his other supporters! We alone were faithful! We alone tried to find him!”
But the boy was trying to fight off the Dementors, even though Harry could see their cold, draining power starting to affect him. The crowd was jeering, some of them on their feet, as the woman swept out of the dungeon, and the boy continued to struggle.
-- HP and the Goblet of Fire, Chapter 15: The Pensieve
What we see here could genuinely be a young and frightened boy, who's spent most of his life with a doting mother if not a loving father, and is facing the prospect of sending the rest of his life in Azkaban. That's enough to make anyone scream and beg for mercy.
Then his time spent in Azkaban, and in captivity at his father's house, changed him even more than he had already been changed by siding with Voldemort. He had no hope of being accepted back into the normal wizarding world; the only person who might accept him was Voldemort, and only if he truly proved his loyalty. He might have been less loyal to Voldemort before his trial, perhaps cozened into being a Death Eater by his older 'friends', and only became such a rabid supporter when it was the only option left open to him.
The ordeals he went through hardened him, and he ended up being contemptuous of those who were unwilling to go through such ordeals in order to prove themselves. How could anyone else claim to be a supporter of Voldemort if they hadn't proved themselves as utterly as he had?