My cats keep fighting !!

I recently got a 2month old girl kitten! My resident cat is 11 years old male. We did everything how they told us first swapped twos no hissing so we moved foward, then we let them smell each other under door and paw at each other. No hissing so we showed them to each other there was some hissing so we stayed their couples days. I gave treats and played throughout, finally they got use to each other i let them smell each other put the kitten in carrier. That was so good we let them free. We had a couple good days ! They would play one of those days they even cuddled.

Idk what happened i think the little girl bugged him too much and now he won’t stop looking for her to beat her up. He doesn’t hiss or growl he chirps and paws out side the door to get in and when we let him in it starts ok then he attacks. We know they arent playing because the little girl hiss’s and he won’t let her get away. It has been two days like this idk what to do now i thought we were good!