First set: Scenario One. His issue is with her. He has no formal relationship with the other man whatsoever, so beating him up is considerably more thuggish and pointless. Beating her up is probably not productive, but technically an interaction with somebody who has a formal relationship with him.
Second set: Scenario Three. His exclusivity is a matter between him and his girlfriend, not him and his best friend, so the same issue applies as before. That said, since they do
have a formal relationship, there's much more room for ambiguity on the matter.
My answer didn't change between the sets, other than to remember that there's a fair amount of cultural wiggle room concerning, well, everything.
In that case my answer would be that they're just as worse as each other in all the scenarios seeing as either way they beat up someone up. I mean, in the first two scenarios, being prepared to harm someone you supposedly loved or just some random stranger you know little about is fucked up either way. In the last two scenarios, either situation involves attacking someone they had some sort of strong bond with.
In all scenario's, Boyfriend is equally bad across each scenario, and worse than the other characters within that scenario, for resorting to violence.
I'll have to echo Empiricist & alexandertnt, then. He's just as much of an asshole across the board.
In all cases, the disproportionate response of "beating someone up" compared to what people did to him, removes any possible justification.
What makes you say the physical aspect is disproportionate, and presumably worse than ending the relationship altogether and refusing to so much as speak to the other person afterwards?