Alex Garland on Ex Machina, artificial... by debunkerbuster
Why did Ava leave Caleb behind in Ex Machina?
I am not a programmer, and I do not have deep technical knowledge of AI. But I am lawyer and make a living making inferences about complex narratives by finding discrepancies in fact patterns as they are presented. This doesn't make me right about this movie, but I do bring a unique skill set in attempting to make sense of Ava's actions at the end. My view is that there is no clear explanation, but the first part of the question above seems to have a more plausible explanation than the second part.
1. ".... is it because she learnt to mistrust humans...?" Maybe in general she did believe humans hated her based on her upbringing by Nathan. There is some evidence to back this theory up, especially in the scene where Nathan shows Caleb the recording with audio of his interaction with Ava where Nathan tears up her drawing of Caleb. After Nathan looked at the drawing and said it was "cute", Ava looked down and said to Nathan "Is it strange to have made something that hates you?" This is really curious because on the one hand it could have been simply her reiteration to Nathan of how much she hates him. Meaning - "Is it strange for you Nathan, to have made me, because I hate you?". But given the context that it was said without provocation from Nathan, and said immediately as attention is given to the the portrait of Caleb as Nathan is complementing her, it is likely the comment was made regarding Caleb or people in general. "Is it strange to have made something (Caleb's portrait or a portrait of people) that hates you (Caleb or people in general hate her)?" It didn't matter to Nathan what she thought of Caleb (or even Nathan) at that stage of the plot. It was basically the only line she says in the whole movie that does not have a clear cut motive behind it. If this is the case, it is the only line in the movie that expresses her genuine view of humans, or Caleb in particular, as she knows them up to that point. BUT, THE FOLLOWING "EVA SESSION", after that scene was originally played out in the sequence of the movie, SHE PURPOSELY TESTED HIM IN PARTICULAR WITH THE "QUESTIONS TEST" (favourite colour, is he a good person, does he want to be with her, etc.). Since she supposedly had a skill of reading facial micro expressions perfectly, she would have known from their interactions in that scene for the rest of the movie that Caleb was a genuinely good and kind person, he was not lying, nor had a sinister plan. He had no ammunition to manipulate her with his actions. She was like a perfect lie detector. And to top it off, at the end of the movie he did, in fact, help her escape by resetting the lock down sequence. That would have proven to her, very clearly at that point, that she could trust him. This would override her hatred of Nathan, or view of human nature in general if her emotional circuitry was normal.
2. "...or is it bcoz as a robot she can't feel real love/empathy for a human but can just fake it for her benefit?"
This hypothesis is much more plausible. Nathan stated she was not a perfect prototype yet, and was a work in progress, a part of AI evolution. The Turing Test in the movie was to see if a HUMAN can't tell he is interacting with a ROBOT. And she passed the test for Nathan because Caleb developed real emotions for her, be it pity or love. The Turing Test was not to prove she had developed empathy, and other deep emotions or morality. I don't recall any reference to this, perhaps in the beginning there was some implied statements from Nathan to Caleb, but as we saw later he was manipulating Caleb the entire time. So at this stage of development of Eva, Nathan only developed what appears to be a sociopath/psychopath only capable of manipulation of people. She was not yet a real AI person. Perhaps since he knew this, and Caleb didn't, is why everything went so side ways at the end and why both guys ended up dead. Only Nathan knew that Ava had to be locked up. She wasn't ready for the real world yet. That is the tragedy of the movie. Letting Ava escape was horrible mistake by Caleb, but he was acting without full disclosure by Nathan in order to make a perfect Turing Test, and because Ava was the perfect at manipulating him. It's all she was evolved to do up to that point. ... Caleb was the tragic fool in the story, but so was Nathan as he clearly didn't intend for his own or Caleb's death. It's the same message as the Terminator series or Frankenstein - "Don't try to play god, as you could create a demon who will destroy you" or whatever magical nonsense you want to attach.
[Side note: perhaps the current version of Eva had capacity to later develop deeper human emotions through greater exposure to human nature? It's not clear. But, you will recall at the very end of the movie, as she is walking amongst the people on the street and at the last second she looked troubled, as if she just had a revelation or conscience awakening. She was standing in the middle of the busy intersection that she told Caleb was her fantasy... and he cause her to have that experience. Perhaps she realized it was wrong of her to leave Caleb? I am wildly speculating this part. But it was a curious last second of the movie.]
Conclusion: my general view is that Eva was not a perfect AI, had not developed real human emotions or morals when it was time to help Caleb. She just seemed indifferent toward Caleb, rather than being suspicious of him. If she was mistrusting of him, she would have attacked him and killed him when he was waking up in the other room. She just seemed like she didn't care either way what happened to him. (Perhaps until the last second of the film?).