Where Kyoko "stabs" Nathan she doesn't really, true. She's come up behind him with the knife out, and while dragging Ava, he literally backs into it.
Pretty sure that's how they meant it to look, as they really didn't do anything in this movie for no reason.
I don't believe they really
wanted to kill anyone, but i think Ava knew it was probably going to be a necessity.
As Ava was trying to escape, Nathan sees her on the surveillance feeds and heads down to prevent her.
By the time Nathan gets down to stop her, she's already had the chat with Kyoko.
She specifically asks Nathan if he is ever going to let her out, and he says yes, which she no doubt reads to be a lie. That's when she runs at him and they struggle.
The point when Nathan answers that he'll let her out (a lie, i presume) is when she likely decided she'd do whatever it was she had to, to get out.
If we're really going to nitpick where he gets stabbed, both from Kyoko and Ava, it appears to be below the heart area AFAIK...
As for attachment, not sure there was any. I think Ava instructs Kyoko on the knife, and Kyoko is fine helping, likely due to being abused by Nathan, or maybe even just because she is used to taking orders, who knows. Like a lot of the movie, it's left up to the viewers' interpretation.
All they needed to do is have the robots escape with the aid of the male protagonist in an intelligent manner and it would have been perfect.
And the common complaint people have rears its ugly head...
Here's the thing, why does she need to take him with her?
Re-watch the movie but looking at it from Ava's perspective!
Caleb is another man controlling her, testing her.
Especially on subsequent re-watches, it seems pretty apparent she is playing him from the beginning in the hopes she can take advantage of his falling for her to help her escape.
And again, looking at things from her point of view, this makes perfect sense.
I also particularly like how during one of their sessions when she's asking Caleb questions, she asks him if he is a good person. When he says yes, she never confirms if he was telling the truth or not. (Not that i think he was lying and is a bad person, but i love how much of the movie allows for various interpretations of what occurred.)
The main problem people have with the movie i think is that they are seeing things from Caleb's view, which is why the ending is so amazing.
You realize he's not the main character the movie is really about at all