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Hero
So, the big question people are asking is if there's any justification for Hulk's sudden surge of team spirit. Sure, he sucker-punches Thor, but he saves Iron Man. That's a big change of heart from almost going Ike Turner on Black Widow.
Remember that Hulk isn't completely mindless/bestial. He's capable of making decisions and recognizing allies--or at least, common threats.
In the Black Widow scene, he changed involuntarily and--this is important--while everyone was still under Loki's influence. Every character was being more vicious/violent than usual at that point.
In the later scene, Banner changes deliberately, he's had a reminder that Hulk can be at least partly controlled (he "aimed" at an old, abandoned building when falling), and there's a clear and present common threat.