The song is about a girl in an abusive relationship (whether it's a friend, boyfriend or husband, it doesn't matter) where she is THINKING of killing him. The killing part isn't really a big part of the song.
It's about this girl trying to make it seem like it's alright, and putting her game-face on and bearing through it all, but over this "facade", she's still in pain.

"Your facade can't disguise the fact that you're in misery."