В каноне нет никаких сведений о преступлениях совершенных Снейпом.

Убивать-да преступление.

т.е. судят и сажают там не за принадлежность к ПС, а за конкретные преступления, убийства, применение непростительных и т.д

Уточнение, он там сгнил за то что применил НЕПРОСТИТЕЛЬНЫЕ. А за это как раз не прощают, о чем и говорит название.
ГП абсолютно противозаконно отделался даже без устного порицания за круцио по Белле и империо по гоблину.

Snape raised his wand and pointed it directly at Dumbledore.
“Avada Kedavra!”
A jet of green light shot from the end of Snape’s wand and hit Dumbledore squarely in the chest.

Imagine that Voldemort’s powerful now. You don’t know who his supporters are, you don’t know who’s working for him and who isn’t; you know he can control people so that they
do terrible things without being able to stop themselves. You’re scared for yourself, and your family, and your friends. Every week, news comes of more deaths, more disappearances, more torturing . . . the Ministry of Magic’s in disarray, they don’t know what to do, they’re trying to keep everything hidden from the Muggles, but meanwhile, Muggles are dying too. Terror everywhere . . .panic . . . confusion . . . that’s how it used to be.
“Well, times like that bring out the best in some people and the worst in others. Crouch’s principles might’ve been good in the beginning — I wouldn’t know. He rose quickly through the Ministry, and he started ordering very harsh measures against Voldemort’s supporters. The Aurors were given new powers — powers to kill rather than capture, for instance. And I wasn’t the only one who was handed straight to the dementors without trial. Crouch fought
violence with violence, and authorized the use of the Unforgivable Curses against suspects. I would say he became as ruthless and cruel as many on the Dark Side. He had his supporters, mind you — plenty of people thought he was going about things the right way,
and there were a lot of witches and wizards clamoring for him to take over as Minister of Magic.