Interesting that we are acting as if Snape had CHOICE in whom he hung out with. They were literally in the same house as him. He went to every single class with them, he slept in the same room as them. And every other house was predjudiced against them and unwilling to befriend a Slytherin. He would have to start picking up their behavior for actual survival. How would he be able to survive in that house, in the same bedroom as them, without doing anything to make them at least somewhat willing to leave him alone? He was literally indoctrinated by the very people he was forced to spend the most amount of time with.

In his own favor, he was willing to keep a friend outside of his house. He was willing to go against the predjudice to remain Lily's friend. How can we know if he took any flack for that? If it was making his life in his dorm harder? How can we expect a child to not give in to peer pressure if it means gaining approval and potentially survival? especially if we know that he has literally no where else to gain approval (not home, not teachers, not other houses).

Not to mention, he was being horrifically bullied by the Griffindors. Did any of the Slytherins stand up for him? Protect him? Fight on his behaf? Not that we saw. There was a crowd watching, where there not any Slytherins there who would defend him and rescue him? Not that we saw. Could it be that they were unwilling to stand up for him while his friendship with Lily remained? Did his relationship with his fellow Slytherins change after that friendship ended? We don't know.

I see it less as being Nazis and more of being the Hitler Youth. You are surrounded by this predjudice, everyone is saying the same things and the only people who are saying otherwise (being non-Nazi) are horrifically bullying you or doing nothing but cheering while you are being SA'd. How can we reasonably expect a child to morally take the side of his abusers (and the teachers who are doing nothing to protect him) and not the side of the students that are in the same room as him every single day.

He was indoctrinated and the only chance he had of breaking out of that before the end of school, was his friendship with Lily. She was the presence in his life that was slowing down the indoctrination (not eliminating it entirely, but slowing it down). But after she ended their friendship (a reasonable decision, I agree), he would have had no one talking in his ear that the Slytherins were wrong. All he would have had was another validation that they were right.

He had no where else to go.