The punch isn't actually the most horrifying thing about this moment. The most exasperating, yes. But the bit that really roots Zerxus to the spot is the fact that Asmodeus lets it happen.

After centuries of knowing he could only strike back when the devil allowed it, and it never meant anything good.

There's a dramatic flair to it, of course; his head rocks back as if the impact actually meant something, and the wheeze could almost be mistaken for pain and not laughter. There's no blood on Steve's knuckles, though; when the blow lands, viciously satisfying for perhaps half a second, it's Zerxus who crumples to the floor.