I'm going to play with my (ex)girlfriend's feelings
What do you mean by playing? Are you trying to hurt her?
Well, I'm planning to treat her like crap when she asks what's wrong with me I wouldn't tell her what bothers me about her.
This doesn't seem like a very good idea to me, I'm sorry to say. It's wrong to mess with others feelings no matter the motivation you have to do it.
She deserves it.
No one deserves having their feelings toyed with. If you have an issue with your ex, be upfront and tell them.