Interesting responses all around. Thinking of it structurally, as the writer/director, one must assume that everything Garland had in the movie was there deliberately. So, when AVA plays "Lie Detector" with Caleb, and her skill in reading micro-expressions is revealed, this must play out later. And it does, as Nathan lies to her about setting her free, and she registers that lie and doesn't hesitate to attack (without precisely knowing how to disable him -- her attack is curiously inept at the outset).
Maybe when she asked Caleb earlier about whether he was a good person, and we don't see her response to his answer, it could go either way -- how does AVA even know what a "good person" is? What could that mean to an AI? Is a good person one who always tells the truth? Is a good person one who always does the right thing? Is a good person one who can be counted on to put your interests ahead of theirs? Whether she's a sociopath or just a ruthless robo-pragmatist shapes her interpretation of what a "good person" is, and which answer matters to her.
If AVA's a sociopath, then "good person" equals one whom she can count on to be able to manipulate -- and Caleb passes this with flying colors, as she completely plays him, as Caleb: 1) puts her interests before his own; 2) falls for her (in every sense of the word). In this case, "good" might as well mean "nice" (or "sucker").
So, when she asks him "Will you stay here?" -- she has shaped the question in a manner already knowing the probability of Caleb's response, based on whatever she inferred from the Lie Detector session.
Although, she halfway miscalculates in terms of syntax, as Caleb repeats "Stay here?" -- he's confused by it, blinded by puppy love as he is, he obeys, just the same. She asks an ambiguously simple question and banks on Caleb's "good person" nature to fill in the gaps she requires.
If she'd given him a more direct command, he may have given her an unfavorable response. I think she phrased that question very carefully, doing a kind of mind trick on Caleb, based on her knowledge of him, knowing that a good/nice person would give the interrogator the benefit of a doubt -- again, putting her interest ahead of his own -- when push came to shove.
I would add this in AVA's favor, however: even though she'd comment to Nathan "Isn't it strange to have made something that hates you?" -- so, she's full-on admitted to Nathan that she "hates" him.
However, in the final fight in the hall, AVA gives Nathan one last chance -- she asks him if she goes back into the box, will he let her free. And Nathan lies to her, so she attacks him.
If she'd been a full-on sociopath, she'd not have given Nathan that last benefit of a doubt. She asked him, he lied, and she attacked.
So, with that in mind, I think her situation with Caleb reflected her having judged him and finding him wanting. Caleb "wasn't her type" (he had failed her in some way, or had outlived his usefulness) -- and in so doing, she had no use for him. It's, as I said, ruthlessly pragmatic, the most cold-blooded dumping in recent cinematic history, but it certainly follows some of her logic.
She asked Nathan that last question, he failed, and she attacked. She asked Caleb that last question, he failed, and she abandoned him -- or else the "last question" was her asking if he was a good person, and her processing that she could manipulate him, and had already written him off. Of course, ironically, it reinforces Nathan's contention earlier that she was only pretending to love Caleb.
But there's a curious moment when she's on her way out of Nathan's man-cave -- namely, that look of spontaneous joy when she's about to walk up the steps. She turns around and reacts with delight. It's very striking, and is honest emotion being expressed. I can't recall exactly what triggers that moment of delight, but I can't help but think that Garland put that there deliberately, to show that AVA is capable of honest emotion -- it's just that she'd not felt honest delight with Caleb (although she appears to have felt honest fear and hatred of Nathan).
You can almost see AVA's processing: Good Person > Nathan. Nathan = Bad Person. Bad Person = Hurting AVA. Good Person = Helping AVA.
There's also the possibility that AVA determined that Caleb wasn't a good person, after all, requiring her to abandon him. He meant something to her, because she does steal a last, momentary glance at him as she's in the elevator -- something motivated that glance.
Maybe it was just to ascertain that he was still trapped as she'd intended; maybe it was because she wanted to remember that moment. But she did steal a glance, and I think, like that delighted smile that follows later, that it was put in there for a reason.
Since Caleb had conceived of a means of trapping Nathan and leaving him to perhaps die (in the original plan), maybe AVA felt that Caleb wasn't so good of a person, or at the very least, posed a threat to her (so she turned the tables on Caleb, not understanding that he loved her, and wouldn't have hurt her).
Or maybe, because she'd murdered Nathan, all bets were off, and Ava was trying to "cover up" her murder because, knowing that Caleb was "a good person," she opted to euthanize him, so he couldn't tell the tale of what had transpired. Again, it speaks more to a ruthless pragmatism than outright malice.