I am not a Torah expert, but I do suspect that it is just not true to claim that the penalty for violating any of the Noahide Laws is capital punishment by decapitation. I have read Sanhedrin 57a and, although I admit it is difficult to understand, I can't see how such a claim can be inferred from this passage.
Some quote Maimonides, Mishneh Torah, Judges, the Laws of Kings 9:14 : can anyone confirm ?
The idea that any robbery should be punished by death - and more specifically by decapitation, looks crazy to me. We all know that there are a lot of antisemitic lies and distortions about the Talmud and I am very inclined to think that this is one of them.
Please excuse my lack of knowledge if I'm wrong.