Perfectly normal behaviour if there are rabbits about, including the eating of the head I am afraid.
Unless you have some nasty diseases in your area it should not cause harm to your cats. But rousing at them after the event is futile I am afraid. They will have no clue about why they are in trouble.
I had a cat once who never realised he was supposed to kill his prey, so he would just bring it home and release it in the house. The baby rabbits I could cope with - the very indignant and aggressive bandicoot required a thick leather ugg boot to remove it
Cats are natural hunters. They will do it no matter how well you feed them. If you don't want dead animals in your house i am afraid the only way to stop it is to keep your cats confined to the house (and even then you get the odd mouse).