The problem with not killing potential monarchs is that their continued existence can be perceived as a latent threat to reassert their divine right to rule at any time in the future.
In my book, assuming the risk of an occasional beheading is part of the job of being a monarch.
I just demanded my neighbor hang my banner on the front of his house, he seems to disagree with your reasoning. Can I give him your phone number so you can explain it to him?