Islam is simply the overarching subject for these people, and it is used to manipulate and indoctrinate many men who have fire in their belly over what they see as injustice. Many will be poorly educated and looking for an outlet and jump at the chance when promised a better life, and importantly promised justice. In some cases you can understand their hate. If your family is blown up by a mis-aimed missile fired by 'the west' you would be angry too. In that regard I do not think you can ever say western foreign policy has not been a huge catalyst to radicalising men into an extreme brand of Islam.
Others will just be trouble makers who like fighting and killing and doing it in the name of Islam is a convenient outlet.
Others will be disenfranchised and manipulated by other Muslims who are operating on an agenda and care not a jot for the men fighting for them as long as they do as they are told (or face the consequences). Indeed we often see this dynamic in gang culture in this country, for example.
Others will just be guns for hire who can talk the talk but are only interested in the end game - how much they get paid and not dying.
ISIS use their particular brand of Islam as a means to an end. Their leaders want their own territory where they make the rules and live like kings. For them to do this, the people in their way have to submit or die and they also need an army willing to die for a cause. Crucially, that cause does not need to be the true cause of the fighting (power, greed etc). If you give someone a purpose and you give them a cause they will die for you have a perfect army. It does not matter if their cause is not the true cause of their leaders, it only matters that they believe it is. In this respect Islam is a perfect tool because it is easy to mis-represent it to those who are looking to belong, who are disillusioned and suffering and who may not have the upbringing and education to question it. ISIS are an example of what happens when anyone with an extremist bigoted/racist/sexist/xenophobic agenda comes to the fore. The only difference is the name shouted when they pull the trigger (For Allah, for King and Country, for the Fuhrer etc etc). As an example, what would happen if the BNP were the total power in this country? They control the army, the police and had an army of angry bigots armed to the teeth and willing to pull the trigger. Would our Muslim population be safe?
What ISIS are doing is no different to what the Romans did, or indeed what the British have done in our colonial days, or what Hitler did. It is a promise of a better tomorrow, and a promise of justice. That is not to say I condone it, but to simply highlight it is nothing new.
The problem is when you have two sides who believe to the death that they are right, you will have war and killing till the end of time until one gives in or is wiped out.
Such is the nature of man, unfortunately. We are both the most intelligent and equally the most ridiculously stupid species on the planet. But it is a tale as old as time itself, and blaming Islam for the acts of those who use it as a tool is short sighted. It is, as it always has been, about the people at the top of their chain getting land, money and power.