Speaking in Downing Street, Mr Cameron said: “David has been murdered in the most callous and brutal way imaginable by an organisation which is the embodiment of evil.  “We will hunt down those responsible and bring them to justice no matter how long it takes.  “David Haines was an aid worker. He went into harms way not to harm people but to help his fellow human beings in the hour of their direst need – from the Balkans to the Middle East.”  The Prime Minister added: “His burning desire to help others has cost him his life. The whole country, like his grieving family, can be incredibly proud of what he did and what he stood for in his humanitarian mission. David Haines was a British hero.”  Mr Cameron chaired a meeting of the Government’s emergency Cobra committee on Sunday morning after Isil extremists released a video showing Mr Haines’ murder.  The Prime Minister described Isil as an “evil” and “callous” organisation.  “They are killing and slaughtering thousands of people – Muslims, Christians, minorities – across Iraq and Syria.  "They boast of their brutality; they claim to do this in the name of Islam. That is nonsense. Islam is a religion of peace. They are not Muslims, they are monsters.”  He warned that Isil are planning attacks in the UK.  “We cannot ignore this threat to our security and that of our allies,”  Mr Cameron said. “There is no question of keeping our heads down that would make us safe.”  The comments are a strong indication that Mr Cameron will in the coming days commit to launching air strikes against the Isil terrorists.  He said he will “degrade and ultimately destroy” Isil with an “iron determination”.  However, Downing Street have said that Mr Cameron is not planning to recall Parliament immediately in response to Mr Haines’ murder.  Mr Cameron made clear that he is prepared to take action "at home and abroad" to counter the threat from Isil.  He also spoke of his shock that the jihadist responsible for Mr Haines' murder - and those of two American journalists - is thought to be a British citizen.  He said: "People across this country will have been sickened by the fact that it could have been a British citizen – a British citizen – who could have carried out this unspeakable act. It is the very opposite of everything our country stands for. It falls to the Government and to each and every one of us to drain this poison from our society and to take on this warped ideology that is radicalising some of our young people.  "The murder of David Haines at the hands of Isil will not lead Britain to shirk our responsibility, with our allies, to deal with the threat that this organisation poses. It must strengthen our resolve. We must recognise that it will take time to eradicate a threat like this; it will require, as I’ve described, action at home and abroad."
The Prime Minister strongly indicates he is preparing to authorise military   air strikes against Isil terrorists in Iraq and Syria as a second British   hostage was named as Alan Henning