She has shown immense patience to you, granted she is immortal, I don't think you need to worry about her changing her mind. If she was willing to honour the wishes of the man she loved as he lay dying, I think you will be fine.

That said, even if you choose not to take the embrace, you should have a relationship with her. She's shown you nothing but patience and understanding. I agree with other posters that blood relations don't necessarily mean anything, but she has certainly done right by you over the years.

If you are that worried about the murder, even if she is killing only bad people, maybe you can try some experiments with her. See how alternatives like blood from a blood bank or otherwise willing donor would work out. If you can come to an arrangement where she won't necessarily need to kill anymore, it might make things more appealing for you too.

EDIT: Also I am no expert, but I don't think vampires always have to fully drain their victims when they feed. Murder is not necessarily the only option.