I'm going to tell my coworker the hard truth
Do you hate them to want to tell them the hard truth?
Most of the time yes, but not always.
It is good to be honest with people but you shouldn't do it with the intent to hurt them. It will make you feel bad to know that you hurt someone.
I agree it's probably better for myself but I want to hurt them anyway.
Won't you still have to work with your coworker? If you try to hurt them won't your workplace relations just be harder in the future? Maybe you should try to have a polite talk with them instead.