Was it really necessary for the plot to make James an aggressor?
Why did the author write the scene in Snape’s Worst Memory to make James Potter so aggressive?
Probably because she wanted to bring a sense of balance and realism to the story.
She had painted Severus Snape as such a monster, it was eventually time to start explaining the Potions Master to us. And because the readership who was originally targeted by the “children’s story” strategy was reaching adolescence and young adulthood, it was also time to make James Potter (and Lily, for that matter) into people to whom we could actually relate, rather than childhood fairy tale-types of parents.
The problem is, when the author made James and Lily more real, it required that we see their human flaws. That’s one of the reasons Lord Voldemort seems like such a caricature to us. He’s difficult to relate to because there’s nothing we can look at about him and think to ourselves, yeah, I’ve been in that situation myself.
With James, we’ve all known someone like him — good looking, gets away with everything, people love him, but when you get to know him, suddenly, you realize, he’s far from perfect and even possibly unlikable to a great degree.
The other problem is, no one wants to admit the obvious — that James Potter wasn’t much better than Severus Snape, whatever our opinions of either of them are.
It’s just a lot easier to continue quoting from the four previous books in which Harry and his pals tell us Severus is one of the bad guys than to consider he might have been the person who was greatly wronged.
We would rather ignore what stares us in the face, which is that there were very understandable reasons for Severus Snape’s animosity towards Harry.
That eye-opener is just so damn disappointing for people like the one who posted this question.
Because it demands we reassess our original opinions of James Potter or else throw common sense to the wind and follow our emotions.
And being emotional creatures, it’s a lot easier to simply continuing to hate Severus Snape, despite the author, in so many words, suggesting we give the character a break.
By the way, that’s the author of the Harry Potter story who wants you to think differently of Snape, not me or anyone else.