she was always going to do it. She was always the monster.
but implicit in that is
Nathan knew it and allowed it to proceed. Doesn't he say that at one point?
Why would Nathan do that?
- he seems to be a man with great zest for life, at the prime of his life, with the world at his fingers. He dances, sings, appreciates art, and all. He enjoys good food. He is in excellent health.
- he seems to have engineered his "retreat" against that, both geographically, in terms of transportation, through the access codes, and even via the power system. This was done at monumental expense because the MD Helicopter has a cruise speed of 154 mph and had been flying over Nathans property for hours - a property that is hundreds of miles across in size is a huge expense anywhere in the world.
- The engineering of Ava's arms and several other parts out of glass - so easily shattered - she is engineered to break easily, to be destroyed. This also speaks to his ongoing self-protective actions even in the recent past.
- he has prior experience with other models, so he has information about tendencies and such. He understands her will, her capability as a threat and consistently and unilaterally acts against it.
- in the end he, though likely drunk or hung-over, grabs the ideal tool and strikes at the vulnerable part immediately, without hesitation, and to large and cruel effect. That cannot be done without preparation and premeditation. Even at that state of the movie, his will to live against the "monster" was strong and intelligent.
- he records them when the power is off. This suggests ongoing distrust of Ava.
The only guess that I have, and it doesn't have backing from the show, and it feels like a very large leap, is that
he fell in love with/gave his heart to an earlier version, then badly destroyed the relationship, and was trying over and over to re-create that her, until he got to the point where he realized that re-visitation was forever beyond his grasp.
The title also suggests it a little in that:
The title of the movie is a kind of play on words is very "textbook" Hollywood. Is it "Ex machina" like ex-girlfriend or ex-wife? Ex can mean "out of" but it can also mean "former". (When I see this I think of Nathan as the former machine given a live heart. I don't want to think of Ava as the former machine who lost her heart but was given sentience and non-machine appearance ... but it fits too well to ignore. )
Is there any other or any good (or both) explanation for why he would do what he did, in engineering the circumstances of her escape, in context of his motivations, prior actions against it, and prior experience with the motivations of the subject? Why would he do that? Why would and did he play that game with Caleb and Ava?