Thor's pride might be calling his shots but it's Bucky's programming that's controlling his. If Thor had approached him differently maybe this could all have ended better but the Soldier is in the forefront of his mind, the importance of the Mission the end all of his existence.

His arm is seized and he makes note of Thor's speed and strength as he ducks under the punch and makes to slam his own fist, the human one, up into the taller mans stomach. At the same time his foot kicks out at Thor's knee while he attempts to pull the metal arm free, the mechanics inside whirring at the strain.