Why is it so important that healthy girls should masturbate to being snuffed?
The word Healthy is important here : a girl with suicidal though, or willing to commit suicide is of no use. (And she should seek help)
What snuff fantasy means in the case of a healthy girl who enjoy living, is that she consider herself as a disposable object. She is to play with, to abuse, to torture, to even break, and when you are done with her, you just get rid of her.
In her mind, her life is devoted to please and satisfy her master(s). Having snuff fantasy means that she belongs totaly to them, and it is normal that they do whatever they want with her, including snuffing her for entertainement or because she is obsolete, or because they have found a replacement.
She is disposable. she doesn't need to be loved, nor remembered. She just want to be used, abused, and thrown away. That's why she has snuff fantasy.