For some reason, it was Ryuk's brow that actually reacted to pain. It creased. And he thought 'I guess that hurt' because he wasn't exactly used to the sensation. It wasn't a good one.

He looked down and noticed he was bleeding. That was weird too. And he was kinda pissed off, actually. That was the main reason he decided to try stamping on the cat. He probably owed Mikami too. But his first thought was revenge.