I know this gets said a lot, but cats tend to live in groups with a pecking order. Some one is going to be the alpha. If it isn't you, it will be your pet. That said, you don't have to be cruel to the animal to establish that pecking order. I have found a squirt bottle full of water to be an excellent way to discipline a cat. If your cat is one of the rare cats that likes to be wet, put a little vinegar in the bottle, enough to make it smell like vinegar. It won't hurt them, although if you put too much in it it may make them a bit nauseated. Now, pick your battles. Pick one behavior to work on first. It sounds like innapropriate displays of aggression might be your top priority. Keep that squirt bottle close at hand,(we kept one in every room for a while) and EVERY TIME the cat is aggressive, warn him and immediately squirt him. If you don't get him within four or five seconds, he won't know why you are squirting him. The cat will quickly learn to cease at the warning. Eventually the behavior will cease altogether. At that point, you can move on to another behavior, such as getting on counters. My youngest cat used to pick on the old cats constantly. Now he will cower and stop as soon as you ask "Do you need to be a wet cat?" It makes for a much more peaceful house. Once that cat accepts that you are in charge of the house and not him, you shouldn't have any problem that you can't handle with an occasional warning or remind squirt.