I'm going to break my ex-girlfriend's heart
Why do you want to break her heart?
Because I know she can get through it and it will make me feel better.
Even if someone is strong and can take it, you should be nice when breaking up. That way you can retain a friend even if you are not dating.
I'm going to take advantage of her kindness by making her think that I like her.
There does not seem to be a reason to do that.   If you 'don't feel that you can remain friends, you can go your separate ways