Answer by Carter Moore:
A few weeks after this question was presented, and many good Quorans gave excellent answers, Islamic scholars released an(ISIS' self-styled leader), in which they eloquently make the case for why ISIS is an affront to Muslims and Islam. Here are what I consider to be the most relevant passages for answering this and similar questions, but I strongly encourage people to read the letter in its entirety.
With regards to Qur’anic exegesis, and the understanding of Hadith, and issue in legal theory in general, the methodology set forth by God in the Qur’an and the Prophet in the Hadith is as follows: to consider everything that has been revealed relating to a particular question in its entirety, without depending on only parts of it, and then to judge—if one is qualified—based on all available scriptural sources. . . . It is not permissible to constantly speak of ‘simplifying matters’, or to cherry-pick an extract from the Qur’an without understanding it within its full context. . . .
When there is a difference of opinion among eminent scholars, the more merciful, i.e. the best, opinion should be chosen. Severity should be avoided, as should the idea that severity is the measure of piety. . . .
In truth, it is clear that you and your fighters are fearless and are ready to sacrifice in your intent for jihad. No truthful person following events—friend or foe—can deny this. However, jihad without legitimate cause, legitimate goals, legitimate purpose, legitimate methodology and legitimate intention is not jihad at all, but rather, warmongering and criminality. . . . The reason behind jihad for Muslims is to fight those who fight them, not to fight anyone who does not fight them, nor to transgress against anyone who has not transgressed against them. . . . [T]here are two kinds of jihad in Islam: the greater jihad, which is the jihad (struggle) against one’s ego; and the lesser jihad, the jihad (struggle) against the enemy. . . . [T]here is no such thing as offensive, aggressive jihad just because people have different religions or opinions. . . .
God says: 'you are not a taskmaster over them' and 'There is no compulsion in religion. Rectitude has become clear from error …' and 'And if your Lord willed, all who are in the earth would have believed together. …' . . . and 'You have your religion and I have my religion.' . . . You have coerced people to convert to Islam just as you have coerced Muslims to accept your views. You also coerce everyone living under your control . . . even in matters which are between the individual and God. . . . This is not enjoining the right and honourable and forbidding the wrong; rather, it is coercion, assault, and constant, random intimidation. . . .
God says in the Qur’an: ‘And do not slay the soul [whose life] God has made inviolable, except with due cause …’ . . . The slaying of a soul—any soul—is haraam (forbidden and inviolable under Islamic Law), [and] it is also one of the most abominable sins (mubiqat). God says in the Qur’an: ‘Because of that, We decreed for the Children of Israel that whoever slays a soul for other than a soul, or for corruption in the land, it shall be as if he had slain mankind altogether; and whoever saves the life of one, it shall be as if he had saved the life of all mankind. …' . . . You have killed many innocents who were neither combatants nor armed, just because they disagree with your opinions. . . . [K]illing prisoners . . . is forbidden in Islamic Law[, yet] you have killed many prisoners. . . .
You have provided ample ammunition for all those who want to call Islam barbaric with your broadcasting of barbaric acts which you pretend are for the sake of Islam. You have given the world a stick with which to beat Islam whereas in reality Islam is completely innocent of these acts and prohibits them. . . . [Y]ou have misinterpreted Islam into a religion of harshness, brutality, torture and murder. As elucidated, this is a great wrong and an offence to Islam, to Muslims and to the entire world.