Who really is behind the killings?

The beheading or other means of executing foreigners who are directly or indirectly helping the occupation of Iraq has continued. The latest victim has been a Turkish truck driver. I saw the execution of this man on Internet:

Someone reads from a prepared statement which is in Arabic. All I can understand from it is is the words God, Islam, occupation, infidels and afew more. Then after the statement ends, the Turkish driver who is sitting in front of the gunmen starts reading from his prepared statement in Turkish. Understanding a bit of Turkish, I realize that he is asking his fellow drivers not to help the US in its occupation of Iraq. Being a Muslim himself, he talks about greatness of Islam and about how the occupation of Iraq is hurting Iraqis.

After his statement ends, he receives three bullets in his head while the sound of Allah-o-Akbar (God is Greater) fills the air.... .

While it was approaching to the last seconds of the clip and to his execution, I could feel my heart beating much faster, I could feel the sudden increase in my blood pressure. I was so nervous and in disbelief I started asking why? Why he had to die? I also felt angry at the Iraqi resistance forces to the occupation. Yes, I was really angry.

..then, suddenly some questions crossed my mind: Who are these killers? Who are the ones who can so easily execute their own fellow Muslim? Are they that stupid not to realize that this can eventually create anger against their own struggle against the occupation and thus, make it even more difficult for them? Is this really beneficial to their cause? Are they not hurting themselves by these killings? Who is really benefiting from all this? and then the big question came: Are they really part of the Iraqi resistance against the occupation or they are deliberately serving the occupiers by tarnishing the images of the resistance to the occupation? And WHO REALLY IS BEHIND ALL THIS?