beheading people for any reason is wrong. There's no rhyme or reason to even consider that to be appropriate behaviour for any action or inaction. Who said that beheading was appropriate action?
The only thing I can say that although I don't agree with, different cultures, laws, beliefs, etc view things differently. So where you or I understand beheading to never be appropriate, there could well be cultures/religions/countries etc that might seem it appropriate...