he is upset, and taking it out on the kitten. Presumably the kitten is a newish member of the household Get some Feliway plugins and get them started off with some feliway spray. At night, keep the adult cat separate from the kitten until he calms down a bit. His inside territory has been invaded by the kitten and the added stress of an outside cat is just pushing him over the edge. I am assuming both cats are neutered. If not the kitten may be reaching sexual maturity which could add to the problem.