Humans are not evolved to be a one-mate species. It may be that even though he has a girlfriend, he finds you interesting and doesn't want to overtly display that.
You shouldn't flirt with him if he has a girlfriend.
You can't possible "hate" him that much to want to flirt with him ...
well if you LOVE him you should
To make her his girlfriend
Yes, of course he can.
I didn't realize that being in a relationship made it illegal to talk to someone from the opposite sex. If this guy is flirting with you, does he do it to everyone? Maybe he is just a flirt. If he is doing it specifically to you either tell him that you are not interested and to stop or tell him to break up with his current girlfriend and start seeing you.
If he likes you, then he won't have the other girlfriend anymore.
You shouldn't flirt because if he already has a girlfriend, it's wrong to try to get him. Just treat him like a normal guy friend, and maybe you can subtly flirt with him. :)
ask him
No it is not a good idea. The guy is not worth it.
He wants to cheat on his girlfriend