My roommate is getting a cat and I am not happy.
Is there a reason you aren't you happy about them getting a cat?
I hate cats
Well, if they got that cat knowing you hate cats, then you're well within your rights to be upset about that.  Roommates should take into account the feelings of those they live with when buying a pet.  Not doing so is super inconsiderate.
I want to kick the cat, but my roommate will be pissed at me.
You have every reason to be frustrated, but kicking the cat would be awful and wrong for reasons beyond your roommate getting pissed off. Don't hurt the cat, but you should discuss the issue with your roommate and tell them that you don't appreciate the cat being in your home and try to come up with a solution.