It's different when the person is also my friend. I wouldn't immediately know who to side with, but obviously I'll worry more of the person who gets physically attacked. If someone is seriously injured, my boyfriend might get into trouble too. In my school you get suspended for fighting, and it might get on your school record. I'd be consoling my friend first so he would feel bad to me if he wants to report my boyfriend. It's also courtesy to take part of the responsibility when your SO did something wrong, so it's completely normal to apologise to the person in behalf of your boyfriend.
If later I talk to my boyfriend and I found out the person has provoked him, then I'll start avoiding that person. But it's only mature to apologise if you used physical violence.
How old are you? No offense, you sound young and immature. Do you think being single is an insult? If you're under 20 years old, hopefully you'll grow up. Dating someone doesn't mean you should be blinded to everyone else. If you put the person you're dating above absolutely everyone else without caring for their opinions, you're obsessive. What will happen if you date a bad person without knowing it? That is a dangerous attitude to have.
What did Shinonome do exactly? Did he curse at Yukimura? Did he immediately ask to break up? He didn't. Who's the one jumping into conclusions here?
I can't blame any of them, honestly.
I understand why Yukimura gets angry, though he shouldn't have used violence but we all lose our temper sometimes, that's human (and he thought Kousaka was stealing his man lol)
Shinonome siding with Kousaka is normal too, look at it physically Kousaka definitely loses to Yukichan, it's unlikely Kousaka attacks Yukichan, it's not his personality. (And if my bf shoves someone onto a table I would be apologetic too, even if I wasn't the one doing it. What's wrong with you ppl? Don't just defend violence be cause it's your bf seriously)
As for Kousaka himself I understand he said what he said because he thought Yukimura doesn't appreciate or love Shinonome and is only using him. If you love someone (even if it's unrequited), and you see that person being treated badly by another person I'm sure you will be upset too. Remember that Kousaka never around to see Yukimura being nice to Shinonome