Because there are crazier people out there than us, who want to ruin it for everyone and take over the world. If we learned anything from WWII I hope we learned to take people seriously when they say they want to rule the world and kill all of group x. So, I will quote them in their own words.
“Those who know nothing of Islam pretend that Islam counsels against war. Those [who say this] are witless.
Islam says: Kill all the unbelievers just as they would kill you all! Does this mean that Muslims should sit back until they are devoured by [the unbelievers]?
Islam says: Kill them, put them to the sword and scatter [their armies].
Islam says: Whatever good there is exists thanks to the sword and in the shadow of the sword! People cannot be made obedient except with the sword! The sword is the key to Paradise, which can be opened only for the Holy Warriors! There are hundreds of other [Quranic] psalms and Hadiths [sayings of the Prophet] urging Muslims to value war and to fight. Does all this mean that Islam is a religion that prevents men from waging war? I spit upon those foolish souls who make such a claim.” — -Ayatollah Khomeini