I wouldn't ever pursue a taken person mostly because girls are fucking crazy and no guy is worth it. You don't know if their partner is crazy and will go after you in a rage of anger or jealousy. Those people exist. Not worth. I'm not desperate enough for that.
If they're in a relationship, they obviously don't like you as much as you'd want them to. There are so many other people out there that will make you just as happy and are just as attractive. Trying to ruin their relationship is just selfish, leave them alone!
No... I have had guys dump their girlfriends for me a few times
It depends how much i liked him. you make it sound like he was one guy in a million, which if that was the case i think i would still try and pursue him. but if it was just a guy i had crush on i would probably not. just wouldn't be worth it