ISIS and David Cameron

Vowing justice for murdered aid worker David Haines, Britain's Prime Minister David Cameron says his killing by extremist group the Islamic State is "an act of pure evil." A video purporting to show Haines' beheading was released Saturday.

That video was released on the same weekend that Haines' family had publicly asked his captors to contact them. Instead, the group released the video, the third to show a Western hostage being killed in the past month.

Calling Haines a "British hero" who was an innocent in the fighting that has consumed much of Syria and swaths of Iraq, Cameron said, "We will do everything in our power to hunt down these murderers and ensure they face justice."

Cameron spoke about the killing Sunday, after meeting with his emergency response team at 10 Downing Street.

He pledged to work with Britain's allies to destroy the Islamic State, saying of the group's members, "They are not Muslims, they are monsters."

Audio from a new video released by the militant group Islamic State (Isis), which appears to show the beheading of British hostage David Haines. In the clip, the Isis militant warns British prime minister David Cameron against arming peshmerga forces in Iraq and supporting American-sanctioned air strikes. The masked man calls Cameron an 'obedient lapdog', and says further actions against Isis will only 'accelerate your destruction'

Despite the fact that the British government has identified the executioner of the two American journalists and of David Haines, it is not making his name public "for operational reasons."

That's because the only likely hope for the remaining hostages is some kind of rescue operation similar to the one that the U.S. Joint Special Operations Command mounted on July 4, which didn't achieve its purpose because the prisoners had been recently moved from the location where they were being held in Syria.

A rescue operation is a long shot, but at this point it may be the only real chance of saving the lives of the remaining hostages. David Cameron knows this -- and so does President Barack Obama.