I'm going to fire a guy who just found out he had cancer
Why are you firing him?
It's not the illness that scares me, it is his laziness. He takes off from work frequently and he can be very frustrating to work with.
If he cannot perform the job to its capabilities than firing him is fine. But you need to make sure that cancer is not impacting his abilities. If it is, then maybe you can work out something so you do not have to fire him.
I don't care about him and he's going to be out on his ass.
If you want to fire him, then that is your choice as the boss. If a job cannot be done by the employee then they should not be employed anymore.