I'm going to tell an ex-fuckbuddy that I don't care enough about him to hate him.
Why do you want to say you don't care about him?
I've just buried the hatchet to the guy yesterday.
While it's good that you attempted to bury the hatchet with an ex, you shouldn't tell him that you don't care enough to hate him as that is very insulting and will just lead to more conflict. You should try to move on past your ex and not interact with him unless it's part of a constructive conversation. Belittling your ex may initially feel good but it just shows that you still actually have unresolved feelings towards him that you need to move past.
I guess I could just leave it alone now.
When you don't care about someone you should really not be spending any time on them.