just forget everything else about the characters and compare James (in that incident) with Dudley. he acted no differently than Dudley does , bullying and humiliating a person in front of many. Dudley uses his wrists as he is good with them , James uses magic as that is his strength.
you see them in a new light that way. we have been hating Dudley and loving James over the years due to the way the story has built up. true James grew up to be a responsible man , but maybe even Dudley would (there is a slim chance though
)
i really disliked James because of that
Even Snape wasnt nice , calling Lily a mudblood when she was trying to help him. but he was worthy of sympathy ,and as i say again,the sympathy is not for that teenager but for what Snape has grown up to be.