The teachings in that book is their justification...Religion??? BS...It's a political movement....
You have just defined an "Islamist" which is, indeed, a political movement disguised as a religion.
An Islamist is a radical Muslim that wants to impose his extreme religious views on the rest of the society and the national Government itself, even if he has to resort to the death penalty to do it.
Islamism: ( is-ˈlä-ˌmi-zəm ) a movement advocating the reordering of government and society in accordance with laws prescribed by Islam .... Webster's Dictionary
Iran and Saudi Arabia today are "Islamist" countries.
Iran under the Shah of Iran was Muslim but was definitely NOT Islamist.
Christian analogies include the Spanish Inquisition, the Puritan Witch Trials and the burnings at the stake in Calvinist Geneva. There are many examples of God endorsing extreme violence in the Old Testament and the contents of the Old Testament were their justifications.
The large majority of Muslims and Christians no longer belong to that religious nutjob fringe. How many Christians or Jews today pay any attention whatsoever to 2 Kings 10:18-27 where the Old Testament advocates the murder of all worshipers of another religion?
However, they still consider themselves either Christian or Muslim.
The Islamists are still back in the day of the 2 Kings 10:18-27 mentality of the 16th Century burnings at the stake practiced by both Catholics and Protestants. Most Muslims have evolved beyond that point but it is always the religious nutjobs that end up on The Evening News on Channel 5.
The secular Turkish Republic has been fighting against Islamists since the 1920's when most of Western Civilization never heard of the term. Yet, Turks still consider themselves "Muslim" just like the Catholic that goes to Church once every 4 years still considers himself "Catholic".
Imagine an English church in the 1970's that decides to respond to Irish Republican Army terrosism by burning Catholic rosaries, burning pictures of the Virgin Mary and hanging effigies of the Pope. The IRA would be laughing their heads of as insulted Catholics, around the world, turned against England.
When you act like a religious nutjob yourself, all you are doing is mud wrestling with the pig and needlessly insulting our anti-Islamist Muslim allies that are fighting, side by side, with our troops in Iraq and Afghanistan against the Islamists that you descibed in your post.