Yeah I'd seriously suspect their motives. Was it their cat that got tortured, did they just witness it immediately before? You know, all the questions you'd want answered about someone who just, say, discovered their spouse in bed with a lover. That would be some serious impulse control issues.
If it was premeditated, and they sought out and killed this person after having decided it was a capital offense to torture a cat, I'd want some damn good answers. Even if they gave a spiel about animal torturers growing up to be sociopaths or something, there could still be extenuating circumstances, and I'd probably end up reporting them to the cops after safely removing myself from the situation.