I'm going to text another man in front of my ex
Why are you going to do that in front of him?
I want to hurt him with it
It can be hard to let go of your feelings for another person even if the relationship ends. There will be more hurt feelings if you continue to try to  hurt him.
I don't care.
You obviously care since you have intentions to hurt them. If they're your ex there is no reason to put them through jealously unless you're actively getting off on it. If someone were to find out you were doing this they'd think you're kind of sadistic.