I'm thinking of potentially screwing a classmate out of a major assignment after he didn't do his work
Why would you do such a thing to him?
I found out he cheated on a test
It's understandable that you are upset about the unequal amount of work that you both put in. However, you should talk to him first and let him know what the consequences will be.
He deserves what he gets, why should I bother talking to him?
Not completing the work for an assignment would equal someone failing. If you talk to him and he does not attempt to make the situation right, it's okay to point out his lack of contributing.