if she gets with you becasue she has been messing around with someone when she already has a bf, then who's to say next time you wont be the bf she does it to.
She's already cheating on her boyfriend and as such is a horrible person.
If you still want her, the only decent thing you can do is tell her to either leave her current bf, or sod off. Imagine if you were him, how would you feel if your gf was cheating on you? Don't be an *******.
Girl has boyfriend. OP likes Girl. Girl likes OP. They've kissed several times.
WAT DO OCUK.
Post a tl;dr version.
Post some pics of her, that's what you should do.
if she hasn't left her BF then she's not that interested in you or she's a ****.
a) She'll leave her boyfriend and be with you until she cheats on you with someone else
or
b) Leave her alone
I think she's only a 'bad' person if she continues to act in this way and play both of you off. She is at a crossroads herself by the sounds of things.
The OP shouldn't be overly forceful, but it isn't douchey to tell her you want to make something of it.