If at that point you didn't care who lived or died in the movie it must have done it's job well as by then there were only two characters left anyway!!!At least you cared about the other 5 beforehand.
I dont think we were supposed to think much of it to be honest, that was the moment i knew not even she'd get out alive, you can't have a hero that's basically condemned someone to their death, even when they arguably deserve it.
I think she did it for both those reasons but the thing that triggered her was the fact that Juno lied to her, she asked her about Beth 'Where is she? Did you see her die?' and Juno lied and said she had when Sarah knew Juno had left Beth to die all on her own. If Juno had been truthful and said, 'i was so freaked out that i'd killed her that i ran off and left her and i feel really bad for it' i think Sarah may not have attacked her.
Another reason Sarah attacked her was because she was going mad(i cant think of a better way to say that), she stopped taking her medication at the beginning of the film, and the title 'The Descent' has a double meaning, it means the descent into the caves and also her descent into madness.
However i like the fact that this film wasn't all cut and dry goodies and baddies because in reality under those circumstances no one knows how they'd react. I'm pretty sure i'd go nuts too...