It doesn't have a short answer though, but let me try. You see extremism originated shortly after the death of the Prophet. Certain Muslims, instigated by enemies in disguise started wars against the rulers believing their rulers to be judging by other than Allah's law. This was the beginning of what we call the "Kaarijites" or extremists. These were the people who killed the Prophet's own beloved nephew & son in law Ali,who was the ruler then cos they deemed him to be a disbeliever! As time wen't on such extremist groups grew. These are people who distort the Quran & misinterpret certain ayahs in it to suit themselves & many of them also disbelieve in authentic hadhits cos they believe their distorted view of the Quran to be sufficient. A good example today being the Al Quaeeda or the "Chechen freedom fighters" Its scary talking to them really. Cos they would even take someone like me to be a disbeliever cos I dont agree with their deviations.Like they believe that rulers of the Muslim lands today & their scholars(including Saudi) are disbelievers cos they let the Americans in n stuff like that!Hence they also believe any Muslim who supports such ideologies are disbelievers too!So its perfectly fine to kill us all.It sounds messed up & it is. There are actually many good books on this subject.where we learn how & what inspired such extremism. We have been warned of it mind you by the Prophet that we should fear extremism & deviation due to it. The safest way is to stick to the sunnah & follow the middle path! Hope I made sense cos like I said its not easy to sum it all up in one post cos the subject is very deep!