I'm going to break up with my girlfriend
Is there a reason you're breaking up with her?
Yes, I hate it when she always complains about everything
People should work on their relationships before ending them but it is understandable that sometimes it is hard to work on them when you no longer like the person. So it is okay instead to end the relationship.
I'm a heartless fuck and I want to hurt her by manipulating her.
Well you shouldn't intentionally hurt her like that. You should just leave her entirely and leave her alone.