I have a cat who, when she was a kitten, played at hunting and fighting all the time. She would growl during this playing and especially during her favourtie game, toast tug of war. Whenever we would eat toast, she would bound up to us and we would play tug of war with a toast crust. She would always growl during this, and would rush up and headbutt us if we stopped before she was done playing.
She is a grownup calm and sedate 5 year old cat now, and I haven't heard her growl in years. The only negative thing left from her fighting habits is that when snuggling she will latch on to my hand and not let go. The other night we fell asleep like that, and then there was a loud noise that startled her and she jumped off the bed, using my hand as a launching pad. Several bandaids were required, as well as a cup of camomile tea to calm my frazzled nerves. That was a heck of a way to be woken up!
From experience, I would take Sandie's advice and not encourage your cat into lifelong hunting and fighting habits. Don't let it cut into your snuggle and play time with the cat, though. That's the most important thing for a kitten.