My cat has recently not been himself. He usually eats all his food but recently he has been leaving some of it (he does go back for it) and he’s had a few accidents out of his litter tray. The only thing that has changed is that I have brought home a new kitten 3 weeks ago but he appeared to be getting along okay. We have been separating them when it seemed the playfighting had gotten a little too much or when the kitten was annoying our older cat Felix but tonight out of nowhere he pinned him down by the throat and it was pretty hard to separate them. he even started hissing at the kitten and me. Any advice on what to do with both him alone and then both together?