But you still killed her. Not only that, but you took part in her being turned into something else entirely. I don't know about Elena, but if I woke up one day as a vampire and had to adjust to a lifestyle that I probably wouldn't have been able to handle, I'd be pretty pissed off. I'd probably try to hurt you in any way that I could.
And maybe being a vampire was too much for her. Maybe she thought your brother was just as bad as you, maybe she thought that killing him would justify what happened to her somehow, maybe she was just that desperate to be human again. I don't know, Rebekah, but as someone who has had a lot of messed up crap happen to them, I think I can see why she'd go at Kol like that. Even if it was wrong.
I can also see why you went after her, too. Same thing. If I had to choose between someone else and saving the people I love? I'd probably do something that I'd regret later on. Doesn't make it right, but there it is.
Sorry to bring it all up, but it was brought to my attention that I needed to know more about everything that happened between the two of you and I wanted to hear it directly from you.