James Potter has a bad rap, and he doesn’t deserve it. Severus Snape has a good rap, and he doesn’t deserve it. The defining moment of James Potter’s character, as far as the HP series goes, is when Harry breaks into Snape’s memories (on accident, of course) and sees his father bullying Snape. Now, don’t get it twisted – I’m not going to defend Potter from bullying Snape. It was a shitty thing to do, bullying is bad, etc. etc.
But! Snape was not just an innocent victim in this scenario. In fact, while he was not the agressor, he was much more vicious – he used the much more vicious Sectumsempra (James used Levicorpus). Not only that, but he waited till James’ back was turned, and he and the other Mauraders were walking away, to attack. This was clearly not about defending himself, this is about revenge.
Kudos to Alan Rickman for making a character who should frankly be despised throughout the fandom so universally loved.