Britain reacted with horror on Saturday to the beheading of hostage Alan Henning, who many had dared to hope might be spared after a cross-community appeal for his release.
Prime Minister David Cameron led tributes to the 47-year-old taxi driver who went to the region as a volunteer to deliver aid and whose death was announced by Islamic State (ISIS) jihadists in a video released late Friday.
"The murder of Alan Henning is absolutely abhorrent, it is senseless, it is completely unforgivable," Cameron said, in comments reflected across the political spectrum.
The father of two was a "man of great peace, kindness and gentleness", he said, adding: "The fact they could murder him in the way they did shows what we are dealing with."
Speaking after a meeting with intelligence, military and government officials on Saturday morning, Cameron vowed to "do everything we can to hunt down and find the people who are responsible for this".
Henning, from Manchester in northwest England, became the fourth Western hostage murdered by IS militants since August and the second Briton, after aid worker David Haines.
But many friends and family had hoped Henning's fate would be different after an appeal for his release led by Britain's Muslim community.
Imams and other influential figures had written an open letter saying that the "despicable threats to Mr. Henning cannot be justified anywhere in the Koran".
And Henning's wife Barbara claimed to have assurances that he had been cleared in a Sharia court of being a spy.
She issued a video appeal to his captors on Tuesday, saying he was a "peaceful, selfless man", and revealed that she had received an audio message from her husband.
The Muslim Council of Britain, the country's biggest Muslim umbrella organization, said his murder was a "despicable and offensive act".
Majid Freeman, an aid worker from the central English town of Leicester who travelled to Syria with Henning, said that "it doesn't make sense to kill him".
"We're all very loud and clear that he's a simple aid worker, he's not gone there for political reasons," he told BBC radio.
Friends said Henning was touched by the suffering of Syria's civilian population, and he joined a group of Muslim friends who founded the charity "Rochdale Aid4Syria".
He was seized in December while taking an aid convoy to the country, and reappeared in the video showing Haines' beheading on September 13, threatened with his life.
Peter Neumann, professor of security studies at King's College London, said the murder made clear that the ISIS organization would not be swayed by outside opinion.
"There's been a massive campaign over the past two weeks by pretty much every facet of the Muslim community in this country to get him to be released, including by a lot of people who would normally be considered extremist," he said.
Henning's murder "clearly shows that they have no red lines", Neumann told BBC radio.
Cameron concurred that "no appeals made any difference", and vowed to use "all the assets we have" to track down those responsible, help other hostages and defeat the IS militants.
But Freeman accused the British government of failing to work hard enough to secure Henning's release.
"The government have done nothing at all to help his case. If anything, they voted for air strikes which may have sealed his fate," the Leicester aid worker said.
British lawmakers voted last week to join US-led air strikes against ISIS jihadists in Iraq, and Royal Air Force (RAF) jets have conducted a number of strikes in recent days.
In the beheading video, Henning explained that he was being made to pay the price of the parliamentary vote.
One opposition Labour MP who did not take part in the vote said she now thought she perhaps should have.
"The vile murderers of ISIL make me almost wish I voted to bomb in Iraq last week," Fiona Mactaggart said on Twitter.
"I want to avenge the death of Alan Henning. I despise them."