In the end, I'm sure Jadis would agree with you: she should have killed Edmund right off. But I'm sure Lewis would use this as an example of how evil blinds itself, and good can use the machinations of evil to its own defeat. It was her greed for power that caused the Witch to overreach herself. She could have done the job with one death, but she wanted to be sure of it and wanted four - possibly to prove to Aslan that He didn't have the power to defeat her. In the real story line, no more than a couple of hours passes between the time that the Witch realizes that her power is broken to the time she decides to execute Edmund. In fact, her dwarf proposes keeping him alive "for bargaining with", but she responds, "Yes! And have him rescued", and proceeds with the execution. Only the immediate arrival of Aslan's people saves him. So Lewis is much more realistic in the book about the Witch's brutality and cavalier attitude toward Edmund's life. I agree with you, though. It was upon their first meeting, at the sleigh, that the Witch made her fatal mistake - she should have killed Edmund then, and I'm sure she came to realize that. But don't forget - Aslan was always in charge!