It's wrong to be bitter.
Xander is at a party where he runs into Kevin, who used to steal his lunch money when they were in grade school.
Xander wants to respond to Kevin.
Xander says hello to Kevin and the two of them have a nice conversation where Kevin ends up apologizing for past behavior.
Xander angrily confronts Kevin about how he was picked on as a kid and the two of them get into a fist fight.
Xander and Kevin strike up a new found friendship.
Xander and Kevin both get hurt and the party is ruined for everyone.