Clearly Bucky's arm is removeable, as prosthetics generally are AND he knew immediately to pick it up and put it back on and how to do that. So I read his expression as less 'omg you can do that?' and more 'omg I didn't think you'd do that' and realising how bad he's fucked up.
He didn't expect Ayo to do that because they have a bond, totally and utterly underestimating how his actions were a betrayal of her and of Wakanda, and that she wasn't prepared to be nice to him about that. When she did that, he realised the extent of what he had done.
The arm is a reminder of what Wakanda, and Ayo specifically, had done for him, of what they gave him, they saved him and gave him this gift, and how he has thrown that back in their face because he felt he knew better.
At the end of the day, Ayo only disarmed his arm/weapon in that moment. She did not destroy it or permanently disarm it, which she could have done, and she didn't take it with her, which she could have done (justifiably - , it's vibranium, Wakanda GAVE it to him).
In a fight situation, his arm is a weapon. She bested him in a fight AND made it clear he knew what they thought about what he'd one. Framing it as pure ableism ignores the context at play, particularly in terms of him being a white man who was given the arm by Wakanda.
Ayo and the Dora were 100% justifiable in what they did, and didn't actually go as far as they justifiably could have done.
However, back to my first point: I don't think it was a kill-switch situation, or that Bucky's expression was shock at her being ABLE to do that. It was shock that she did it, and realisation of what it meant for their relationship (and his relationship with Wakanda).
Sidenote, but the Dora utterly destroying John Walker was a thing of a beauty that I will never stop watching.
Side-sidenote: I feel like everyone going 'it was a kill switch and Bucky didn't realise you can do that' maybe don't know how prosthetics work (i.e. that they're removeable and not permanently grafted)?
Addition: a lot of the commentary on this issue by us, as white people, is also totally overlooking the fact that this is a white man who was saved by the people of an African nation, the arm was made of their resources, and he used it to free the man who murdered their king.
Maybe we should step back in being angry at Ayo and the Dora and listen to Black fans on this? Just a thought.
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