Good actions don't actually cancel out previous bad actions (obviously) but they're better than doing nothing or continuing with bad actions.

The Stannis Baratheon crowd is going to hate me for this.
But honestly despite my being an incredibly vengeful person I can be swayed by people acting good in the moment. And the amount of good Snape did far outdoes the amount of bad. Not only that but he suffered for the bad he did and genuinely repented. The idea of having somebody pay harshly for their crimes does appeal to me, but pragmatic "The greater good" justice outdoes that.

I think he was unnecessarily cruel to Harry for the most part but he did so much to keep him alive and safe. And he was risking his own life to be a 'double agent', always working for the 'good-side'. He died doing so.

Well the thing is,even if he did do a lot of good things,they still don't erase the bad. The most they can do is help us overlook them.

no, i really dislike when people make snape out to be a misunderstood good guy.
he may have done some good things but they are far out weighed by the bad things, the good doesn't cancel out the bad in any way.
yes he had things happen to him that sucked, however that doesn't change his choices, he chose to become who he was, and who he was was often unforgivable.

^Exactly.

He was still pretty dang abusive toward the students, and tormented Neville every chance he could. And plus, doing good things doesn't go back in time and prevent the bad from happening. They happened. He did terrible things. He did great things to make up for it, but the sweet doesn't cancel out the bitter in his situation.