British PM on New ISIS Beheading: 'They're Not Muslims, [But] Monsters'

Aid worker David Haines apparently executed.

Cameron spoke hours after a video emerged online that appeared to show the murder of British citizen David Haines at the hands of a black clad ISIS militant. Before his death, Haines reads from a scripted statement, saying Cameron is "entirely responsible" for his impending murder.

Today Cameron said the U.K. would not be cowed.

Haines’ brother, Michael, issued a statement through the British Foreign Office about David who he said “was recently murdered in cold blood.”

“David was like so very many of us, just another bloke,” Michael Haines said. “David was a good brother, there when I needed him and absent when I didn’t…He helped whoever needed help, regardless of race, creed or religion.

“David was most alive and enthusiastic in his humanitarian roles… He was and is loved by all his family and will be missed terribly,” Michael Haines said.

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Haines was previously seen at the end of the Sotloff video, ISIS implying that he would be next. Similarly, another hostage appears at the end of this new ISIS video.

President Obama issued a statement Saturday saying the U.S. "strongly condemns the barbaric murder."

"The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve," Obama said.

In addition to the second hostage shown at the end of the video, ISIS is believed to be holding at least one other Westerner, a 26-year-old American woman.

Military special operations veterans told ABC News that in addition to the overt message delivered by Haines and his purported killer, the circumstances of the video’s production could be signal to Western intelligence. By shooting in broad daylight and in open desert, the officials said ISIS could be saying that it feels safe from the long reach of Western and allied militaries and intelligence.