The ISIS video post showing Haines' beheading called his execution "a message to the allies of America."
It is produced very similarly to the videos that showed the executions of American journalists James Foley and Steven Sotloff, the last of which included Haines and the threat that he'd be killed next.
The new video pictures a masked ISIS militant placing his hand on another captive, whom he identified as Alan Henning, a British citizen.
In a tweet, British Prime Minister David Cameron called "the murder of David Haines" an "act of pure evil."
Cameron added, "We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."
Haines offers brief scripted comments on the video, as does the man who kills him.
Directing his remarks at Britain, the executioner -- who sounds like the man who killed Foley and Sotloff -- says, "Your evil alliance with America, which continues to strike the Muslims of Iraq and most recently bombed the Haditha dam, will only accelerate your destruction and claim the role of the obedient lap dog.
"Cameron will only drag you and your people into another bloody and unwinnable war."
Retired Lt. Col. Rick Francona -- an Air Force veteran intelligence officer and CNN military analyst -- surmised that if ISIS planned to dissuade Britain for teaming up with the United States, the group will be disappointed.
"ISIS has just guaranteed British cooperation with the American on all phases of what we're going to be doing," Francona said. "... I think this is now a Western fight; it's not just a U.S. fight."
Haines' family just sent public message to his captors
News of the gruesome killing came the same day that Haines' family released a brief message to his captors through the British Foreign Office.
In it, the family says, "We have sent messages to you to which we have not received a reply. We are asking those holding David to make contact with us."
A logistics and security manager for the Paris-based Agency for Technical Cooperation and Development, a nongovernmental humanitarian agency, the 44-year-old Haines was abducted in March 2013 near a refugee camp in Atmeh, Syria.
At that time, Haines was working to arrange for the delivery of humanitarian aid to people staying at the camp. He had previously worked on aid operations for victims of conflict in the Balkans, African and other parts of the Middle East, according to an ACTED spokesman.
British hostage of ISIS was helping displaced Syrians, aid group says
Haines' face became known to the world in the ISIS video, released September 2, in which he looks forward and kneels as a masked ISIS militant stands behind him.
The militant says in that video, "We take this opportunity to warn those governments who've entered this evil alliance of America against the Islamic State to back off and leave our people alone."
British officials said after the video's release that they had sent troops to try to rescue an unidentified British citizen "some time ago," but failed. They released no other details.
ISIS, which also calls itself the Islamic State, has killed thousands in Syria and Iraq as it presses a military campaign to establish itself as an Islamic caliphate. Witnesses report mass killings, beheadings and crucifixions.
The United States has been using airstrikes to support Iraqi and Kurdish forces battling ISIS, and is working to build a coalition to broaden the effort against ISIS. U.S. Secretary of State John Kerry was in Egypt Saturday seeking that country's help in the fight.
John Kerry seeks Egypt's support for mission to build coalition against ISIS