Cat Keeps Pinning Kitten Down

Why does my male cat kep pinning down and biting the neck of my female cat.
Cat keeps pinning kitten down. I have a 6 year old male cat named rusty and a 6 week old female kitten named luna. 3 months ago i brought a new kitten home she was a kitten from an alley cat i i quarantined her for 2 weeks while inside vet visits with new kitten nikita. He follows her literally everywhere she goes and jumps on her then starts biting her neck.
A trip out of your cat s comfort zone like an appointment at the veterinarian can result in cats with their ears pinned back. Lately rusty had been pinning luna down and biting her neck making her cry. Cats usually deal with intruders by trying to avoid them and only display overt aggression as a last resort.
He keeps biting the kitten on the neck and sometimes on the genitals and pinning him down. I have a one yr old cat and today we got an 8 week old kitten they re both female the cat took to her straight away was licking and cuddling up to her and has been doing that since 10 this morning but just now i caught her biting the kittens neck throat and belly and trying to pin her down its not very hard but its harder than playful biting why is she doing this and should i be worried she. Luna is very playful and follows rusty everywhere.
Rusty has been around other animals before he grew up with a dog and he used to live with another cat. He doesn t let go until i make him. Natasha is my older cat about 7 yrs old.
She hisses and growls and tries to run away from him but he keeps persisting until we have to take him away from her. Provide your older cat with areas outside the kitten s reach where only it can go. I adopted a kitten 3 months old and my other cat 9 months old keeps aggressively attacking him.