Her name was Katherine, and we didn't know what she was at the time, she played us both for fools, telling us each in secret, letting us each think she'd told us alone. It didn't take us long to find out, of course, and in our arrogance we demanded she choose, instead she faked her own death and we killed each other over her, not knowing until years later, after she nearly killed us again, that she was still alive.
Damon's fights are his own, they always have been. Especially the ones he starts. For that matter, I kill no one who hasn't tried to kill me first. But if you come for him instead, I will defend him. He's my brother and as often as we disagree we're all we have left.