I'm not gonna pretend like I understand the science behind a lot of it, but basically she's got a ton of identical copies of her running around, and the people she grew up with taught her they were bad and she had to kill them. I guess she saw Cosima's post, ambushed her and dragged her into one of the empty cabins. By the time I got there, she'd already strangled her.

I don't think she usually gets that up close and personal with a kill, but I definitely wasn't letting her borrow my rifle any time soon.