Fear. Dark, abject fear of the unknown, of demons, of powers conspiring against one. The same fear that has scarred the African subcontinent for millenia. The only way to alleviate such culturally ingrained fear is to take action against those you believe are in league with your enemies. By killing or mutilating such hapless people and thereby "proving them weak", these fearful man/boys hope to reestablish their own courage and manhood. But of course it never works.
The motivation is demonic, in a sense - for it springs directly from an excessive fear of the supernatural by the uneducated.
Fear always springs from ignorance.
Ralph Waldo Emerson