TL; DR: É especificamente porque ele é um ser da lógica. Além disso, por causa do amor.
Primeiramente, é importante lembrar a declaração original de lealdade da Vision em Age of Ultron . Ele vai ficar do lado de qualquer grupo que proteja a vida da melhor forma.
CAPTAIN AMERICA: Are you? On our side?
VISION: It's not that simple. (pause) I'm on the side of life. Ultron is not.
Ele explica que vê a lógica na supervisão (todas as citações seguintes da transcrição da Guerra Civil ):
Vision: In the 8 years since Mr. Stark announced himself as Iron Man, the number of known enhanced persons has grown exponentially. And during the same period, the number of potentially world-ending events has risen at a commensurate rate.
Steve Rogers: Are you saying it's our fault?
Vision: I'm saying there may be a causality. Our very strength invites challenge. Challenge incites conflict. And conflict... breeds catastrophe. Oversight... Oversight is not an idea that can be dismissed out of hand.
Ele ama Wanda. O ator Paul Bettany confirma isso em uma entrevista com EW . Falando de por que ele acidentalmente explodiu Rhodey do céu, ele diz
“What happened there is that his judgment was clouded there by real love and affection,” says Bettany. “And he responded in a quick and thoughtless way.… I really love that the moral compass of the movie is this synthetic person who’s trying to figure out what it means to be human. It’s a neat idea, you know?”
Porque ele ama Wanda, ele acredita que essa é a melhor maneira de protegê-la.
Tony Stark: If we don't do this now, it's gonna be done to us later. That's the fact. That won't be pretty.
Wanda Maximoff: You're saying they'll come for me.
Vision: We would protect you.
Ele realmente acredita que proteger Wanda do público também protege o público.
Wanda Maximoff: Vision, are you not letting me leave?
Vision: [He blocks her way.] It is a question of safety.
Wanda Maximoff: I can protect myself.
Vision: [He holds her arm.] Not yours. Mr. Stark would like to avoid the possibility of another public incident. Until the Accords are on a... more secured foundation.
Wanda Maximoff: And what do you want?
Vision: For people to see you... as I do. [She looks at him gravely.]
e
Vision: [He faulters.] If you do this... they will never stop being afraid of you.
Então, ele decide seguir a rota utilitária: o maior bem para o maior número:
Vision: Captain Rogers. I know you believe what you're doing is right. But for the collective good you must surrender now. [Tony's team arrives.]
Claro, sendo que ele está baseando sua lealdade na lógica, ele está disposto a trocar de lado.
O ator Paul Bettany falou brevemente sobre isso na entrevista mencionada anteriormente.
“He has logic in abundance. And logic does not afford any room for loyalty. New information could come to light and he might flip to Captain America’s side. But love solidifies things and provides loyalty. So if he’s there to protect mankind – or frankly, the universe – he needs to figure out what’s good about these creatures, and I think that’s the quest that you find him on. And, of course, in the comics, the relationship becomes romantic. We’ll see moving forward what happens for Vision and Wanda.”