Sharia is the law that regulates the daily life of a Muslim and serves as a guide for living by Islamic principles. As Islam is not only a belief system but a complete way of life, the law covers all aspects of living including moral, spiritual, intellectual, physical, economical, political, etc. Sharia law is derived by scholars through interpretations of Islam's canonical texts, the Qur'an and Hadith (sayings and actions of Muhammad). As with any legal system the interpretations can range across the liberal-conservative spectrum, and opinions on the meanings and applications will often differ. Therefore sharia law is essentially an effort to comprehend God's instructions and apply them in daily life. Since the interpretations are made by humans, they are subject to error and even perversion. This occurs when unqualified, ignorant and/or corrupted individuals make the interpretation. For instance the KKK perverts Christian texts to conjure up false rulings. The same can be said for certain Muslim groups and so-called "Islamic states" regarding Islamic texts. While there is no Pope in Islam to serve as the authoritative interpreter, there are recognized institutions and scholars whom analyze, discuss, deliberate and arrive at reasonable rulings through a process known as "fiqh" (Islamic jurisprudence). This nuanced and sophisticated science operates on the premise that the Islamic texts are fluid and dynamic; that anything outside of the very basic tenets can, and should, be interpreted according to the particular time, place and culture. So if any version of sharia advocates brutality, injustice, extremism, terrorism, etc., the question must be asked: are such rulings endorsed by mainstream Islamic scholars, or are they being posited by those devoid of a true understanding of the religion, or the wisdom to apply it?
Some attribute the early and rapid spread of Islam to forced conversions by the sword. While it is accurate that the Muslim empire initially spread, for the most part, through battles and conquests (a common phenomenon for that time) the religion of Islam itself was never forced on anyone who found themselves living under Muslim rule. In fact, non-Muslims were afforded the right to worship as they pleased as long as a tax, called "jizyah", was paid. During the Dark Ages, Jews, Christians and others were given protection by the Muslims from religious persecutions happening in Europe. Islam teaches no compulsion in religion (Qur'an 2:256 and 10:99). For more, read "The Spread of Islam in the World" by Thomas Arnold.
Terrorism, unjustified violence and the killing of non-combatant civilians (or even intimidating, threatening or injuring them) are all absolutely forbidden in Islam. Islam is a way of life that is meant to bring peace to a society whether its people are Muslim or not. The extreme actions of those who claim to be Muslim may be a result of their ignorance, frustration, uncontrolled anger or political (not religious) ambitions. Anyone who condones or commits an act of terrorism in the name of Islam is simply not following Islam and is, in fact, violating its very tenets. These people are individuals with their own personal views and agendas. Fanatical Muslims are no more representative of the true teachings of Islam than fanatical Christians are of the true teachings of Christianity or fanatical Jews are of the true teachings of Judaism. The most prominent examples of such “religious” fanatics are Anders Behring Breivik, the 2011 Norwegian terrorist who claimed in his manifesto to be “100 percent Christian” and Baruch Goldstein, perpetrator of the 1994 Hebron massacre who is considered by some Jews to be a “hero” and a “saint”. Extremism and fanaticism are problems not exclusive to Muslims. Anyone who thinks that all Muslims are terrorists should note that terror groups like ISIS (or ISIL), Al-Qaeda and Boko Haram kill Muslims as well. Also, the former boxer Muhammad Ali, perhaps the most celebrated person of our era, is a practicing Muslim.
The word "jihad" does not mean "holy war". It actually means "to struggle" or "to strive". In a religious context it means the struggle to successfully surrender one's will to the will of God. Some Muslims may say they are going for "jihad" when fighting in a war to defend themselves or others, but they say this because they are conceding that it will be a tremendous struggle. But there are many other forms of jihad which are much more relevant to the everyday life of a Muslim such as the struggles against laziness, arrogance, stinginess, one's own ego, or the struggle against a tyrant ruler or against the temptations of Satan, etc. Regarding the so-called verses of "holy war" in the Qur'an, two points: A) The term "holy war" neither appears in the Arabic text of the Qur'an nor in any classical teachings of Islam. B) The vast majority of verses in the Qur'an pertaining to violence refer to wartime situations in which Muslims were permitted to defend themselves against violent aggression. Any rational, intellectual analysis of the context and historical circumstances surrounding such verses, often ignored by pundits or violent extremists, proves this to be true. Other verses of violence deal with stopping oppression, capital punishment and the like.