He's not fantasizing about he himself beheading atheists. What he's fantasizing about is subtly different: he's fantasizing about the idea that atheists will get beheaded because of their own atheism rebounding on them, so it's their own fault.
Robertson doesn't strike me as a particularly scholarly thinker, but even less well-thought religious folk have confronted the problems of evil and tragedy. The story of Job is a common subject of discussion in churches and among religious folk, and it's always framed as horrible things happened to Job because of his belief in a deity and because of the deity. Christians aren't unused to the concept of bad things happened because of their faith rebounding on them.
He's fantasizing about the outside world giving 'indisputable proof' of external morality. ...
Link to Blog Post: "Extremism in Thought Experiments is No Vice"
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This is a LW discussion post for Yvain's blog posts at Slate Star Codex, as per tog's suggestion:
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