I was in a similar situation. This dude, i really really liked him, he was so cool and so sweet to me. Him and his girlfriend were doing that whole on again, off again thing, but she was always around. And i seriously did think about pursuing him until i saw how he treated his on again off again girl. He would be rude to her, ignore her, and take all she did for him for granted. So i had to separate myself from my emotions and use some logic to realize that it wouldnt be right for me to talk to him. Mainly its wrong because he never made a clean break from his girl, so even when they're technically "broken up" shes still there. And if i did end up with him, what would make me any different from her, what would stop him from treating me the way he treats her??
And i strongly believe that everything you do WILL come back to you in one way or another, so i wouldnt not want to jeopardize any of my future relationships by coming in between someone else's...And i'm happy i made the right decision because i would have just ended up gettin hurt