That was the correct choice for this situation. For both of them, honestly. Mettaton was so aggravated he was beginning to overheat himself, and besides, he had no idea what sort of abilities Kaworu might have. Clearly he wasn't human or else he wouldn't behave like a psychopath who doesn't know when he's insulted someone.

Once the boy's out of his sight, Mettaton relaxes. He was angry, that's exactly how he started, but with Kaworu gone, that righteous fury switched to such paralyzing guilt that he just put his back to the wall, closed his eye and tilted his head up as if he could somehow drown out his emotional upset this way.

He doesn't move for a very long time, and if anyone crosses his path again, he will outright ignore them. He's done talking for the day.