I thought that Nina had to kill Teri because Teri had overheard Nina speaking in German. The person Nina was talking to said something like "don't let anyone know you're connected with germany" or something like that.
Originally posted by DarkAngel
When Teri walked in on her and saw the dead body, that was problematic for Nina. I don't think she knew for certain whether Jack was alive or not. The plan was that Drazen would kill him. If Nina left Teri alive and ran, someone might find Teri, and she would tell them Nina had done this. Killing Teri would prevent her from telling anybody anything, giving Nina more time. Of course, that didn't happen, since Jack killed Drazen and showed up to stop her.
I might have forgotten some of the details, but that's what I recall.
Hello. I came to talk. I've been thinking lately. About you and me. About what's going to happen to us, in the end. We're going to kill each other, aren't we? Perhaps later. I just wanted to know that I'd made a genuine attempt to talk things over and avert that outcome. Just once... I don't fully understand why ours should be such a fatal relationship, but I don't want your murder on my ... hands...
- The Killing Joke