I'm not convinced. Germany did start two world wars and the monarchy has had its share of lunatics.
If inbreeding is a root cause of insanity, killing your own child for not marrying her cousin is at least circumstantial, if not prima facia evidence.
Though to believe such dogma one could conclude it's insane...to someone living in the 21 century instead of the 8th century...to a 8th century bearded savage it's just the way of Islam....
Though, I do get your point...