I want to make out with this dude's ex in front of him
Why would you want to do that?
I was just following my heart and if he gets upset about it he should get over himself.
You shouldn't do that in front of him. Do it in private or just stay away from that girl.
Why? Isn't that best revenge?
It only hurts yourself to try and get revenge on other people. Instead of trying to make this guy mad, and potentially confusing his ex with your intentions in the process, you should just let whatever anger you're holding onto go and move on with your life.