Beheading isn't legal in the UK or America, probably not in most normal countries.
It's the sort of behaviour one would expect from a fanatical hostile sub division of a group.
Human instinct may want to see such video's, be honest, have you never trawled Google looking for something, iffy.
Personally, I think this kind of video should be barred from facebook.
There's plenty more places to go and watch such stuff.
Is Facebook too big. Can't they control the content anymore. But want the profits