Once the cat lord has been generated friendly, no amount of cat killing will make him hostile. Killing a cat, even in front of him, then chatting with him will still result in the RotMC (although the game summary will include the "fell from grace" part of the achievement).

Less importantly (although I'd still say it should be fixed), eating a cat corpse in front of him will also not make him hostile, whether the corpse was obtained by killing a cat or otherwise.