Erwin without hesitation launched himself at Klaus. Klaus with his arms out hoping to stop Erwin knew it was only going to end up being a futile attempt. He already knew by then that Erwin when fully committed was like a freight train. Stopping him would simply be like trying to dodge the drizzles from a downpour.
But fortunately, Erwin didn't go overboard this time. After payback by landing a good five shots on Klaus's face with his fist, he has reached satisfaction and felt that the "judgement" passed was just appropriate. The entire time Klaus's face was being pounded on, Klaus did nothing to retaliate nor protect himself. He just stood and received it all. In hindsight, that was even the reason Erwin had left him alone.
