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A popular belief from s1 is that Jackson beat up exer because of what he did to pamela, however that's not actually right
If you go back to read prior to the fight you'll realize that Jackson was never really pissed off about what exer did until he realized that exer been sabotaging him too, that's when he gets angry.
Of course he acknowledges that's what exer did to pamela and yes it upset him and he believes pam should know; but the thing is that it doesn't upset him enough to beat exer up
Do you know what upsets him enough?
Pamela's makeover and joining the reds
This is what upsets him enough to beat exer up
Why?
Because looking at this from a different perspective; it looks like exer is still trying to sabotage Jackson's life and isolate him.
The only people on Jackson's side are pamela and brenda, but brenda is more of with both sides and with David being her brother and she obviously regaining her crush on exer; that would leave Jackson with only one person he can absolutely trust won't leave him: Pamela
So seeing Pamela with the reds felt like everything was crumbling for him; that exer was manipulating pam to join him and his friends and then exer would continue his cat and mouse game that Jackson now knows of.
So really it wasn't about pamela as a person as much as it was about pamela as Jackson's last remaining companion in that rivalry thing