Somebody posted this on another thread but I found it interesting..Ava said to him "Can you wait here," which may not have been a question but a challenge. Not sure if this is what the film maker intended but your question is an interesting one and requires an equally interesting answer

It both confused and manipulated Caleb, it perhaps gave him the impression she would come back for him. But she didn't.

Here's the way I look at it. Assume Ava had far superior intelligence, as depicted, she would not kill for the sake of killing if she could accomplish her goals in a different way. Through planning and cunning.
Caleb