But you say yourself that believing it is right to fight nonbelievers for the sake of Islamic rule is a credible reading of the Quran. We know this because you say the Quran "certainly does have passages which encourage this sort of behavior".
So why would you then say anyone who follows that interpretation "clearly has something very wrong with them"? It is their entirely credible (by your own admission) interpretation of a book containing what they believe is the immutable words of God.