How can I get even with someone who hurt me?

I just don't understand some men. They hurt. They are cynical. They lie. They are s. This guy I met was a great catch, until he showed his real self. We dated several times and then he disappear. He played with me 'cause I discovered that he has a 7 years relationship with this other girl, and now he calls me and tells me he miss me, but he doesn't tell me the truth. World is so small... This girlfriend is a friend's secretary, so now I can see a way to relieve my thirst for revenge, but... I don't know how! I don't wanna hurt her, she doesn't even know I exist. And the cheating won't admit she exists. So? What to do?