My Character's not doing what I want her to?

I'm writing a book and one of my main character's is a girl called Mo (she's not the center character but she plays a big part) She's a thief and an in and doesn't trust easily. And always looks out for herself and doesn't like to risk her own neck. I was going to have her help subtly then disaper but lately my mind keeps telling me to kill her. It's like she wants to die doing one great deed. But I really don't want to kill her off because she's one of my favorite characters. What do I do?