Lots of societies have indulged in decapitation, notably including the Irish and the French.
Here’s a discussion of the possible history of the method in Mexican drug wars.
http://www.time.com/time/world/article/0,8599,1839576,00.html
Apparently rare prior to 2006. One possible explanation is that the thugs saw the Islamist videos and thought, “Cool!” Another is influence from MS-13 and other Central American gangs.
So did the English prior to the Glorious revolution.
As for looking for cultural reasons why people turn into savages,people are people-you need no other explanation.