I also think that there is no excuse for Sophie to behave like that. Yeah she had hard things in past, poor girl, of course. But there isn't anything that holds her now from changing to good side, there is no any "brother" that points gun to her and says "go be bitch and slut". She has every means and possibilities to just sit, think and reflect about it, and then to start changing little by little. She doesn't need some goof-boy lecturing her in alley on subject "what is love baby dont hurt me", she is capable of figuring it all by herself. Well maybe changing your lifestyle without friends (even slightly stupid as MC) is harder, but still.
And judging by this alley-speech, she actually fully understands, that her life is messed up and she wanted it to change. But still lying there and bleeding. Therefore let her lie there as long as she wants, back home we have a really nice girl, who loves us and dont try to make lick her after good "brother-fuck".
So for me Chris' ending is the best too, and this friend-harem-newridiculouslife-end - is just overweight and unnecessary altruism and moralfagging.