This is a declaration that those who aid unbelievers are themselves unbelievers, and can thus be killed. This is a well-attested principle in Islamic law. From the Jerusalem Post:
A lengthy audiotape purportedly by the spiritual adviser to Abu Musab al-Zarqawi’s al-Qaida-linked group justifies killing Muslims when their presence protects infidels and threatens revenge on Shiite Muslims.
The 74-minute tape, posted Wednesday on an Internet site that often carries statements from Islamic terrorists, identifies the speaker as Sheik Abu Anas al-Shami, spiritual leader for Tawhid and Jihad. His name often is cited in Islamic forums as a religious authority, but he was not known to have released such audiotapes.
“If infidels take Muslims as protectors and Muslims do not fight them, it is allowed to kill the Muslims,” the speaker said, then gave an example.
“If there is an interest in killing a Turk, and if it is necessary – meaning that if targeting the infidels is impossible as long as the Turk exists – then killing the Turk serves an interest to all Muslims and his killing is allowed,” he said.
Turks, Jordanians, Egyptians and other Muslims have been abducted, but thus far no Muslims are known to have been decapitated.
On extremist Islamic online forums, the accidental killing of Muslims who work for US coalition forces in Iraq has always been a controversial subject, but generally it has been considered justifiable.
Opinions have been more mixed on taking Muslims hostage and beheading them, with some saying “fellow Muslims” should be spared and others urging their killing as examples to others of the fate of those who are “allied with the devil.”