That part I know. What I don't know is why. I don't know why there's a group of people killing others, seemingly for no reason.
Islam isn't one hive mind. There are variations of Islam such as Shia, Sunni, Sufi etc. All are Muslims, but their interpretation of Islam varies slightly with differing levels of implication, much like if you want to discuss Christianity or Catholicism, both are similar but have small differences with small or larger implications in their teachings.You can break down Christianity further and there are loads of different variations (Mormons, Lutherans, Protestants etc etc).
All follow the same basic structure but a number of differences exist; some through interpretation, others appeared later for political reasons. ie. King Henry VIII could not divorce his wife who failed to give birth to him a Son to take over his throne, the Church advised him he could not simply divorce her, so he chucked a shit fit and decided fuck it I'll make my own religion where you can divorce and viola, Church of England was created.
One of the main differences is that Europe and most of the Western World have had time to have their wars, get it all out in the open and come to some sort of agreements over land, power and politics and as such time to set aside differences and move forward. Islam is a bit over 600yrs younger and has had plenty of outside interference coming in to pull power in different directions.
Islam began to split approximately 100yrs after Mohammed, again the differences are subtle but enough to divide the Muslims in some key ways. Like most religions the branches mostly divide over differences in interpretations and implication particularly when it comes to power and politics. Don't forget religion is spread by word and teachings and Arabia at the time (and to some degree still is) very tribal, some tribes took on one variation of Islam, other tribes another. So a country isn't necessarily all one or the other, countries may have a fairly balanced number of believers of each denomination while some have a majority of one or the other. The civil war in Iraq is a largely sectarian one that has existed for hundreds of years. Sadam was a Sunni and so the Shia muslims being a minority were largely kept quiet or otherwise oppressed (and you were really fucked if you were Kurdish). Then Sadam went and a new Government came in that was led by a Shiite (Shia), at the time many of the Sunni were fearful that there would be a large backlash against them from the Shia having been largely oppressed during Sadams regime and they were not just being paranoid because there is enough there to suggest the new Shia government hasn't exactly been quick to denounce or even admit the existence of so called 'Shia Death Squads' who basically want the Sunnis out. For some of the Sunnis, ISIS probably looks like a pretty good deal when compared to the Shia government currently in place.
Thats not to say that Sunni and Shia sects can't get a long, it's just to say that when you have a look at most of the conflicts coming out of the middle east, it's largely a Sunni vs Shia one. There are plenty of areas where Sunni and Shia get along, but theres also many where they don't and thats where the problem is.
Islam isnt just a faith, it is an all encompassing structure upon which it speaks of how to live your life but also covers things like law, governance, politics etc, the interpretations of which some believe allows for growth and progression so the needs of the people, the laws and governance should reflect that and adapt as necessary. There really isn't any room for that in Shariah. As far as Shariah is concerned, this is what it says in the Quran, therefore it shall always be, hence you have some loonies that still think stoning a women to death for adultery is the status quo. Such behaviour wasn't unusual for that time period, it was still all the rage for a lot of Christians etc. It's just that we've moved on from that but there are some out there like ISIS, multiple fuckwits in England, various other little hotspots around the world etc. that can't seem to pull their head out of their ass and get with the times.
I'm being very broad in this with little specifics but yeh, ISIS follows a form of Islam that doesn't necessarily reflect the same belief of all Muslims, hence why ISIS, seemingly oddly to many, are killing Muslims also. But Shiites and Sunnis have been killing each other for a long time, it's nothing new. What is new is that ISIS is probably the most extreme group to have ever emerged from this sectarian violence in our lifetime, maybe ever. They don't seem to believe in compromise with anyone, it is their way or death and they have been particularly proud in demonstrating that they will kill anyone and everyone that doesn't fit their particularly brutal lifestyle. If they could get their way, everyone would be indoctrinated into their form of Islam and those would didn't convert or weren't already would be killed or sold into slavery.
Though it has also become rather interesting to see that ISIS members either aren't quite as close to the faith as they make out or that their ranks are increasingly becoming filled with angry young men that just want to go out and kill as they have displayed unusual behaviour not in practice with any real form of Islam in some instances.
But mostly it can be summed up that ISIS is full of fucktards.
