That is due to politics and foreign meddling, and has nothing to do with Islam. Although I concede that Muslims killing Muslims is forbidden in Islam, but allowed when one party attacks another one, the rest should gang up against the attacker.
I am pretty sure that you understand the role of politics in these fightings and killings and not Islam, meaning, using politics to manipulate Muslims against each other. I think the problem is education and inequalities, since most of these fighters you see are composed of mostly non-educated people, which represent a big pool in the Muslim world (around 15 %), and who are easily manipulated and of some disillusioned people who can not see any better future for them, which is the new trend.
But it is all related to governance and politics.
Islam as a political system although not politicized in government, was fair, just and equitable for all its citizens, hence brought peace and prosperity to all of them, So it is peaceful in that regard.
Any civilization or society who has deviated from equality and fairness for its citizens has paid a price for it.
This is why I maintain that Islam is peaceful and pacifying, since that is its aim for the individual as well as for the society.
This is an explanation which is very good excuse to cover up the shortcoming..If these when terrorist organization does something favorable to one party then it is for Islam, when it is not favorable to you then you can term as politically used by Non Muslims....Classic example is Taliban,Let,ALQ and ISS...