I didn't take it as literally...
It seems more like, she doesn't really approve of whomever her boyfriend is dating and she upsets her, but only because of how much she loves her brother and wants the best for him and no one will ever be good enough for him in her eyes.
She knows no way to fix her killing... she doesn't know how to start being nice to the girl and feel okay about their relationship, but she knows she needs to because he loves this girl.
Now that I've read everyone else's I'm not sure, and maybe if you look at the whole album it's different but this is how I first interpreted the song.
I didn't take it as literally... It seems more like, she doesn't really approve of whomever her boyfriend is dating and she upsets her, but only because of how much she loves her brother and wants the best for him and no one will ever be good enough for him in her eyes. She knows no way to fix her killing... she doesn't know how to start being nice to the girl and feel okay about their relationship, but she knows she needs to because he loves this girl.
Now that I've read everyone else's I'm not sure, and maybe if you look at the whole album it's different but this is how I first interpreted the song.