Would it be okay to pretend that I'm going to help him?
For what purpose?
I'm going to destroy them both, financially and emotionally
That sounds creepy and despicable. You should get some help before you hurt someone, wind up in jail -- or both.
I don't want to make the mistake of trusting a woman again. I'm going to destroy him like I promised.
You shouldn't be a predator who is pretending to be nice with the goal of destruction. A lot of people would look down on that and consider it very creepy and weird, is that the kind of reputation that you want to have?