The cat may have taken it's anger out on Mikami, which was probably a bad thing, but it was angry, so Ryuk's job was done, really. He'd achieved what he'd personally set out to do.

He was about to aim another kick, for fun and to avoid any bitching from the human, when the guy surprised Ryuk and decided to try stabbing at the creature. He'd honestly thought these guys only had the balls to kill by writing a name and keeping their hands clean.