"Rabastan's cats did liven up the common room," he says, smiling a little. He'd appreciated the company of the cats, if not their master. The cats would sit on him while he studied, and since Rabastan and he never spoke to each other if it could be avoided, nothing had ever been said about it. If a minor coup, it still counted. He smirks. "Not then. But, you see, just that tiny image--one he couldn't be certain of--and he knew he could never be innocent again. And I knew he knew."
"You've been mad since before I we met," Severus informs him with a quarter-smile. "'Mad, bad, and dangerous to know.'"
He looks surprised, but doesn't comment. "And you doubt my classroom competence," he sniffs. "Now you're just shielding him because he brings you free beer."
He thinks about it. Was he disturbed? "I'd argue that," he says finally, "but it's not important. If what you mean is did he impress me as being dangerous, certainly. However, I'm not at all certain he'd be interested in the unsuspecting."