I guess they think one of them would get my power if I die. But you brought up a good point. For all we know, the part of the legend that I'm born with my powers isn't true, just as long as I have those powers. And suppose the witches feel like my power belongs to them, it could be that none of them are suitable of this kind of power. I mean the roach monster told me my power wold be too great and perhaps they wouldn't be able to handle it if they had it. Of course, they'd think my powers rightfully belong to them and thus they want to steal it and, for good masure, kill me.