Religion.

One extra sad thing about the ISIS and Hamas and Taliban and Al Qaeda extremism which is running rife at the moment is how it is being used to attack 'all religion'.
Even in the days of the Crusades, a millennium ago, I doubt that the worst excesses saw people butchered all that frequently simply for being of the wrong shade of belief. Catholicism has not got clean hands and, at times, Protestantism was less than perfect (consider Oliver Cromwell) in Tudor and Stewart times but for half a millennium, such barbarism from fringe Christians has been happily rare but we are all found 'guilty by association with violent religion'.