His intro was him being a dick to Peter, he was being a dick to a kid at lunch and then beat the shit out of Peter for lulz and then was being a douche to a random girl in the gym.
Now, given how badly Flash beat him up and how often he attacked other people, I personally feel Peter would have been somewhat justified in fighting back physically, I know i would have, but he didn't. Peter was the bigger man and stopped at just trolling him in basketball. As a result of Peter defending himself, combined with Peter slamming against the locker, Flash learned to respect Peter and stopped bullying him and the two made amends off screen. Walking away solves nothing. You just need to stand up for yourself and push back.
Watch I Am Number 4 if you want to see the results of just trying to walk away.