I think 'us vs them' is perspective.
I think even 'evil' knows when it is being evil.

I think Voldemort knows he is doing 'bad/dark magic'. I think he also defines his actions as evil. I think he continues to do so, because it is about ultimate power and compensation for having a childhood lacking in parental love and guidance. Upon finding people like himself, Voldemort found himself to be a natural leader.

I think Harry is a good counter-argument to 'circumstances' define who we are. Harry - also - had a difficult entry into life. But he choose to define himself through good deeds. Harry didn't have much problems with people his own age.

Snape also wouldn't have had an ideal childhood. I believe his parent(s) were ok towards him, but his Hogwarts years weren't so kind. Snape I think had difficulties with his peers.

It is interesting in the ways we choose to defy others around us.
Voldemort - by exploring dark magic. it's cool to be the 'bad boy on the block'. I think there would have been a lot of pride and reputation about a boy (Tom Riddle) who took magic where it had never yet gone before.
Harry - by trying to find his own identity beyond his legacy which took him a long time to come to terms with. I think this is perhaps the main reason for his rule breaking. Finally learning WHO he is. knowing why he didn't fit in with the Dursleys... Forever trying to figure out how he survived the killing curse. Feeling obligated to saving those, just as he was saved by 1. Lily, 2. Dumbledore, Hagrid and the rest of the original Order of the Phoenix, 3. his friends - Ron, Hermione, Neville etc on countless occasions and 4. even by Snape.

While on the subject of Harry - he didn't have kids with Ginny on the day the Great War ended. It was a few years later. Plenty of time for an adult to digest everything they had learned, seen and experienced. I do think it genuinely possible Harry AND Ginny would have named their son Albus Severus. Just as WE discuss the pros and cons of Snape, Surely Harry, Ron, Hermione, Ginny, Neville, Luna etc etc would have done so....

Snape - by joining the Death Eaters (who were also Slytherins) but also studying hard (withdrawing further into himself). I think Snape's fondness for Lily would not have happened if he had met the Deatheaters first. thankfully Lily knew snape before all the issues of peer pressure and keeping up appearances started to take hold.