ISIS Extremists Claim to Behead British Hostage David HainesBYPHIL HELSELISIS Video Purportedly Shows Hostage David Haines' BeheadingNIGHTLY NEWS Terrorist group ISIS released a video Saturday purportedly showing the execution of British aid worker David Cawthorne Haines.If confirmed, it is the third time the organization, which has seized large swaths of Iraq and Syria, beheaded a captive and released a video of the execution, intending to send a message to Western nations. Britain recently said it wouldsend machine guns and ammunitionto Iraq to help that government battle the militants, and it is weighing whether to participate along with the U.S. in possible airstrikes in Syria against the terror group.Haines, 44, like others in taped executions by the terror group, is forced in the new video to read a script in which he blamed his death on British Prime Minister David Cameron and the British governments decision to join the U.S. in its battle against ISIS.A black-clad, masked militant holding a knife then says the execution is in retaliation for Britains decision to supply arms to Kurdish fighters battling ISIS in Iraq.After the murder, another captive is shown and the executioner warns Cameron "you ... will have the blood of your people on your hands, if the U.K. continues to ally with the U.S. in its campaign against the group.Cameron called the execution "an act of pure evil.""This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil. My heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal," the prime minister said."We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes," Cameron added.President Barack Obama said, "The United States strongly condemns the barbaric murder of UK citizen David Haines and offered his condolences to the family and the country.The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve. We will work with the United Kingdom and a broad coalition of nations from the region and around the world to bring the perpetrators of this outrageous act to justice, and to degrade and destroy this threat to the people of our countries, the region and the world, Obama said in a statement.Haines, a father of two, was reported abducted in early 2013 in Syria while working for an aid group.His brother, Mike Haines, said in a statement that David was "murdered in cold blood." He remembered the former Royal Air Force aircraft engineer and humanitarian worker as "just another bloke" and a "good brother" who adored his children."David was most alive and enthusiastic in his humanitarian roles. His joy and anticipation for the work he went to do in Syria is for myself and family the most important element of this whole sad affair," Mike Haines said in a statement on behalf of the family. "He was and is loved by all his family and will be missed terribly."The statement said David Haines participated in United Nations missions in the Balkans, and those experiences made him want to pursue humanitarian work.ISIS released a video Sept. 2 showing the beheading of American journalist Steven Sotloff, who was kidnapped in Syria in August 2013. Another American journalist, James Foley, was executed and a video was released Aug. 19.