Ibn Khaldun, one of Islam's most respected philosophers, understood that "the holy war is a religious duty, because of the universalism of the Muslim mission and (the obligation to) convert everybody to Islam either by persuasion or by force", many other Muslims are either unaware or willfully ignorant of the Quran's near absence of verses that preach universal non-violence. Their understanding of Islam comes from what they are taught by others. In the West, it is typical for believers to think that their religion must be like Christianity - preaching the New Testament virtues of peace, love, and tolerance - because Muslims are taught that Islam is supposed to be superior in every way. They are somewhat surprised and embarrassed to find that this is contradicted by the Quran and the bloody history of Islam's genesis.
Others simply accept the violence. In 1991, a Palestinian couple in America was convicted of stabbing their daughter to death for being too Westernized. A family friend came to their defense, excoriating the jury for not understanding the "culture", claiming that the father was merely following "the religion" and saying that the couple had to "discipline their daughter or lose respect." (source). In 2011, unrepentant Palestinian terrorists, responsible for the brutal murders of civilians, women and children explicitly in the name of Allah were treated to a luxurious "holy pilgrimage" to Mecca by the Saudi king - without a single Muslim voice raised in protest.
The most prestigious Islamic university in the world today is Cairo's al-Azhar. While the university is very quick to condemn secular Muslims who critique the religion, it has never condemned ISIS as a group of infidels despite horrific carnage in the name of Allah. When asked why, the university's Grand Imam, Ahmed al-Tayeb explained: " Al Azhar cannot accuse any [Muslim] of being a kafir [infidel], as long as he believes in Allah and the Last Day—even if he commits every atrocity."
For their part, Western liberals would do well not to sacrifice critical thinking to the god of political correctness, or look for reasons to bring other religion down to the level of Islam merely to avoid the existential truth that this it is both different and dangerous.
There are just too many Muslims who take the Quran literally... and too many others who couldn't care less about the violence done in the name of Islam.