What should I do if I'm developing feelings for a girl that has a boyfriend?
Why would you want to steal another's girl?
I don't want to steal her, I just want to start a side relationship with her.
Trying to get a person to cheat on their partner is manipulative and emotionally damaging behavior. You should not do anything that negatively affects another relationship. Rather than pursue someone who is taken, focus instead on dating someone who is single and available.
She is into me. I can not resist her, she is beautiful.
I hear what you are saying about her being the chaser, however, that does not mean you have to engage. You should let her know that she ought to respect her relationship or end it completely if she is serious about you.