I can't believe you revived this thread for this issue, Wolfgang! But since you did, here is my response.

I disagree with Wolfgang because, taking a historical position, there have been times and places when decapitation was much more common. Paris in 1799, for example, and other times when it was a recognized form of execution. Therefore our hypothetical foreign astronaut, if he were also a time-traveler, could look at various historical documents and find out exactly where to go if he wanted to observe large numbers of humans being decapitated by other humans. If you tried to do this with lions you would fail, because there is no such time or place.

I agree that it is rare. So is the juggling of bowling balls. But we would not say "juggling bowling balls is inhuman." And I WOULD venture to say that humankind "semi-regularly" engages in the juggling of bowling balls; that is, it is not unbelievably anomalous, just a relative rarity.

My point is this: to say "this behavior is inhuman" merely enables us to sweep it aside and not bother to try to understand it so that we can combat it. It's like the people who say "the terrorists bomb us because they hate freedom." It's essentially meaningless.

The behavior was human, because humans did it. Moreover, humans perform this particular kind of behavior (decapitation) more than most other species (except some insects and arachnids, as Beardedbruce points out). I don't want to suggest that other animals wouldn't if they could; chimpanzees have been observed "at war," for example, but you can't decapitate someone with a stick.

Decapitation of a member of one's own species is something that few other species have the wherewithal or the occasion to do. Humans do it far more often. Thus if we need to decide if it is "human" or "inhuman," an emprical, rational, historical approach would make it necessary for us to conclude that it is more human than inhuman.

Like juggling bowling balls, this does not mean it is very common, or acceptable, or morally good. But if we want to stop it, it's better to look for its causes in human psychology than to claim it's so alien we can't possibly understand it.