-He was a magic nazi and only stopped because a girl he liked cried. He didnt stop his prejudice though

-obssessed over a girl who was nice to him a couple times and didnt accept that she didnt have feelings for him.

-was an asshole to an orphan because his mother didnt reciprocate his feelings

-was an asshole to a kid whose parents were tortured into insanity because he blamed him partially for the death of the girl he likes because neville could have been the child of prophecy instead of harry

-reported lupin for being a werewolf which allowed umbridge to pass a bill to systematically oppress werewolves

-Held resentment towards his fellow professors because he kept getting passed up for being DADA instructor due to being an asshole and made no effort to change to be a proper instructor

Theres probably more that im forgetting but this is what i can remember off the top of my head. Snapes an irredeemable asshole that made no effort to be a better person. his working against voldemort was purely for selfish means.