Could I tell a guy that I would be a fwb then decided not to?
Why are you going to lead him on?
Because I wanted to be able to reject his sexual advances without hurting his feelings
You shouldn't lie to someone about being in a FWB relationship. That will hurt his feelings, and it could also make him angry because you deceived him.
It didn't hurt his feelings and I want him to think I'm a fucking terrible person...
You shouldn't want people to think you are a terrible person. It's bad to go back on your word, but if you don't want a sexual relationship, you should be honest about that, and you shouldn't feel forced into one. Try to be an honest person so that people see you as a good person instead.