I'm going to play loud games on my computer at noon while my roommate is still asleep
Won't that bother your sleeping roommate?
I can do whatever I want when he's sleeping.
You should respect the fact that your roommate is trying to sleep. Try to be quiet and go about your business without disrupting their sleep.
What if I do it anyway and he walks in on me.
I just think it's best if you avoid that situation entirely and let him sleep. You can play on the computer at any other time.