The last guy was pretty obviously going to screw them if they let him live, but since we didn't see what this guy did in a flashback, I guess we just have to take protag's word that crippling him here was the right call.
This is like schoolchild that gets picked on going to school with a gun and shoots up the place. The MC is literally murdering the schoolyard for minor temporary slights. Everyone calls him trash but he could just go and spar the strong people and this rumor would be instantly destroyed.
They bullied him so much they killed the real bodies owner. I don't see how taking revenge isn't called for, even if he was a normal college student. Remember his memories merged so... idk