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
![ROFL :rofl:](https://www.purrsinourhearts.co.uk/Smileys/purrs smilies/rotf.gif)
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).