What might be causing my cat's violent behaviour?

Rabies is probably not involved here. Once the infection has started causing erratic behavior, the behavior doesn't abate, or come and go. If anything it escalates without mercy until the animal is unable to stand steady or walk without stumbling around and falling down. And yes, your cat would probably be comatose or dead by now if he had contracted rabies a few weeks ago. The rabies vaccine is pretty darn effective, so if your cats are up-to-date on their vaccines, you're good as gold.

Intact males will sometimes tolerate male kittens, but it sounds like your grey cat has reached an age that is making the black cat feel like his territory is threatened. The grey kitten was probably like a toy for the older black cat, but now that it's "come of age", the black male is most likely feeling insecure. It's really best if you neuter the black cat as soon as possible, for everyone's sake. Afterwards, he'll remember what a cool buddy the grey kitty is, and they'll go back to being friends.

I'm really concerned though, about the injuries you've been sustaining. It prompts me to write some cautionary notes about handling cats, ANY cat, in the future. When a cat is in the middle of a fight -- and the fight is already in progress the minute the growling and posturing has started -- regardless if it's your own cute snuggly furbuddy or a scar-faced scrapin' neighborhood feral Tom, please NEVER use any part of your body to intervene!! Brooms or water are the best fight-stoppers. Brooms are used to gently shoo them apart, not to hit with (for readers who might get the wrong idea), and a squirt of water in the face is always a good, humane method as well. Cat scratches and bites can end up being pretty bad injuries -- with some not so nice diseases associated with them -- so any time you do get scratched or bitten and the wound becomes red and/or hot feeling and/or swollen or you get a fever, you should have a doctor take a look at it right away.

Hope this helped and good luck!

Get him neutered! A sexually mature male is extremely territorial - spraying is one sign of that, becoming aggressive is another. At this point, I'm surprised he isn't spraying all over the place, so consider yourself very lucky. Call the vet today and get him in to be neutered as well. It will only get worse, I'm afraid.