I don't want to sit through a graduation ceremony
Who's ceremony is it?
It's my uncle's graduation ceremony, I don't want to have anything to do with him because he used to mistreat me when I was a kid.
If that's the case, I can understand your point of view. Don't attend if you really don't want to.
It doesn't matter if I feel bad. My uncle will feel bad that he didn't treat me well in the past and he will treat me better in the future.
You shouldn't wish him ill will, but you certainly have every right to distance yourself from him.