http://www.nationalreview.com/articles/243580/its-about-sharia-andrew-c-mccarthy

Some Islamists employ mass-murder attacks while others prefer a gradual march through our institutions — our legal, political, academic,  and financial systems, as well as our broader culture; the goal of both, though, is the same. The stealth Islamists occasionally feign outrage at the terrorists, but their quarrel is over methodology and pace. Both camps covet the same outcome.

That outcome is the death of freedom. In Islamist ideology, sharia is deemed to be the necessary precondition for Islamicizing a society — for Islam is not merely a religious doctrine, but a comprehensive socio-economic and political system. As the former speaker elaborated, sharia embodies principles and punishments that are abhorrent to Western values. Indeed, its foundational premise is anti-American, holding that we are not free people at liberty to govern ourselves irrespective of any theocratic code, that people are instead beholden to the Islamic state, which is divinely enjoined to impose Allah’s laws.

Sharia, moreover, is anti-equality. It subjugates women and brutally punishes transgressors, particularly homosexuals and apostates. While our law forbids cruel and unusual punishments, the brutality in sharia sanctions is not gratuitous, but intentional: It is meant to enforce Allah’s will by striking example.

Violent or not, have very good reasons for the wanting to destroy the West.  Islamists devoutly believe, based on a well-founded interpretation of Islamic doctrine, that they have been commanded by Allah to kill, convert, or subdue all who do not adhere to sharia — because they regard Allah as their only master (“There is no God but Allah”). It is thus entirely rational (albeit frightening to us) that they accept the scriptural instruction that the very existence of those who resist sharia is offensive to Allah, and that a powerful example must be made of those resisters in order to induce the submission of all — “submission” being the meaning of Islam.