It may probably happened once to you, to like a girl that is going through a bad relationship. This girl usually complains about her horrible boyfriend and she says about him that does everything wrong. She probably told you that you are different and you are the one that is always there for her. The thing is so obvious, she would like to be with you instead.
She doesn`t leave this relationship because deep down she doesn`t want to do so. She thinks that she is not ready. This happens because she is still attracted to him. She has some romantic feelings towards him. She`s probably missing him as an emotional and stable connection. You are there to fulfill her emotional needs. In the meantime she will continue to have sex with her boyfriend that she hates.
You need to know that she is not the victim you think she is. She will always run back into his arms. She is equally responsible for the drama relationship she is in. If she fears for her safety, she is probably the victim of abuse. When this is happening she has a lot of people in her life that tell her to move on. You should stop being a knight in shining armor. You won`t be turning her on in a romantic way. Attraction doesn`t work like that. This is just an emotion she needs to feel when she is around you. Attraction is built by flirting.
You should ask why you need to choose her. Would you like to have a girl that stays with a man who treats her like crap? This is exactly what she does, she cheats emotionally on him. Would that other guy would be fine knowing that his women is having an emotional relationship with other guy. You are usually chasing this girl because you like to settle for less. You don`t have a high level of self-respect. Another reason why you are chasing her is because you think that she is hot. You also probably think that you won`t get the chance to be with a girl as physically attractive as this one. The thirds reason why you chase this girl is that you are protecting your ego. You have invested a lot in this girl and you can`t bear to lose her. You have to win, otherwise you will see yourself as a failure.