I'm going to not let my roommate eat my pizza
Why won't you share your pizza with your roommate?
He tried to take a slice without asking first and I caught him in the act.
I understand that. It would be one thing if he asked you first, but he did not. It seems wrong to share if he was willing to steal.
Yes, but he is my friend.
It is up to you if you want to share or not. I can understand if you don't want to share with your friend because of what he did. It is your choice.