Don't kick yourself, its very common to be confused because cats play fairly roughly and especially when there is a size differences and some protest noises it can be hard to be sure what is going on if you haven't seen it before. But yeah, like all predators, cats practice hunting, fight and defense behaviors in play. So you'll see play bites on the neck, which is the kill move in the wild. You'll see them rake each other with their back legs -- the classic defensive manuever when a cat is attacked in the wild. And of course stalking & pouncing. Even the wrestling is practice for fighting other cats, as there is a lot of rolling around in real cat fights. So it all looks kind of violent, but its good fun for them.