If they adhere to a point of view which is in the minority among Muslims -- in response to a text which, due to being inherently contradictory, can be interpreted in multiple different ways -- they are "radical." That's not a defense of Islam; it defines the term in general. Radicals are people who adopt the less popular view of something, then go to extremes to promote it.

In addition, ISIS isn't following the letter of the Koran in regards to those passages to which they claim to adhere. They make a lot of token gestures, which are far less inconvenient than the actual requirements presented. They do this by defining for themselves what the behavior required to do things like "purify oneself" actually consists of.

For example, an ISIS leader previously claimed that his taking female captives as sex slaves was acceptable, because he purified himself before and after having sex with them. He did this through prayer and brief fasting. Sometimes, he was raping multiple women throughout a given day.

The whole experience with a single instance, purification included, took up less than a day. Meanwhile, elsewhere in the Koran, there are several days' worth of rituals and behavior required to ensure said purity. Additionally, he was flexing the definition of what constituted an acceptable female captive.

This kind of requirement starkly limits the amount of such activity that can occur, which is thought by some to be the reason behind its inclusion; the pre-Islamic tribal societies of the Middle East saw this behavior occur in a far less controlled, far more regular fashion. If you live within a society which permits rape in unrestricted fashion, and it is acceptable by the morals of that society, and you implement a rule which says "okay... you may rape women. But, you can only rape women in this particular circumstance, and you have to do this stuff for two days beforehand, and this stuff for two days afterward," that's progressive of you in the strictest sense of the term.

It's not particularly pleasant by modern standards, but it was a starkly evident improvement over what existed beforehand.

As society advances, religion needs to advance with it, and adapt to changing standards. Instead, what ISIS is trying to do is to go back on some of the improvements made to the pre-Islamic tribal society by the Koran. This is why many Muslims look at their actions, and refer to them as extremists, radicals, or false. Speaking personally, I would still say that it's their mess to clean up, but I don't believe that ISIS stains all of Islam any more than Christianity stains me for what some of my relatively recent ancestors did.

I don't follow Islam, or any other religious order, but I find it curious how often people who claim that there is no such thing as a higher authority are prone to impose modern morals on an ancient world, wherein everybody understood things to work differently.

Either there is a higher source for morality, or what is generally accepted within a contiguous society at a given time is acceptable. Meanwhile, what is generally acceptable in Muslim society today is rule through fearmongering.

I've heard the same statistics that the rest of us have; things like "one third of all Muslims believe that death for leaving the religion is acceptable." If I were living in the Middle East today, and someone I didn't know asked me that question, I'd say "yes, absolutely," because the people who disagree with that sentiment aren't the ones who are going to kill me for answering in the fashion that they consider wrong.