Jeremy Corbyn has been criticised by his shadow ministers as "naive" after saying he would have preferred it if Jihadi John had been taken alive.
The British Labour leader echoed a campaign group once labelled "terrorist apologists" after he said it would have been "far better" if Mohammed Emwazi had been "held to account".
In marked contrast to David Cameron, the Labour leader also failed to praise the intelligence agencies and Armed Forces for their work in tracking down Emwazi in Raqqa.
One shadow minister said that Mr Corbyn's response, nine hours after news of the strike became public, demonstrated that he has "no comprehension" of the situation in Syria.
Ian Austin, a Labour MP, said: "Look, why couldn't the police just go and arrest Emwazi? It's not as if it's a really dangerous war zone and I'm sure he'd have come quietly."
Cage, an organisation that had links to Emwazi, claimed he had been the victim of a "state-sponsored assassination" and that he was entitled to his human rights. The group caused controversy in February when it tried to blame MI5 for Emwazi becoming an Isil killer.
The Prime Minister confirmed yesterday morning that Emwazi had been targeted in an "act of self-defence" that would "strike at the heart of Isil".
Using Emwazi's name for the first time, Mr Cameron said that Britain worked "hand in glove" with the US to track down the Londoner.
Mr Cameron said: "Emwazi is a barbaric murderer. He was shown in those sickening videos of the beheadings of British aid workers. He posed an ongoing and serious threat to innocent civilians not only in Syria, but around the world, and in the United Kingdom, too.
"He was Isil's lead executioner, and let us never forget that he killed many, many, Muslims, too. And he was intent on murdering many more people. So this was an act of self-defence. It was the right thing to do."
He praised the intelligence agencies and the Armed Forces for their "extraordinary work" during the operation and thanked the US, saying that Britain has "no better friend".
In a warning to other British jihadists, he said that "we never forget" those that pose a threat. He said: "If this strike was successful, it will be a strike at the heart of Isil. And it will demonstrate to those who would do Britain, our people and our allies harm: we have a long reach, we have unwavering determination and we never forget about our citizens.
"The threat Isil pose continues. Britain and her allies will not rest until we have defeated this evil terrorist death cult, and the poisonous ideology on which it feeds." He said that his thoughts are with the families of those who were "so brutally murdered", including British aid workers David Haines and Alan Henning. Mr Cameron said: "Their families and their friends should be proud of them, as we are. They were the best of British and they will be remembered long after the murderers of Isil are forgotten."
By contrast, Mr Corbyn's response was three sentences long: "We await identification of the person targeted in last night's US air attack in Syria.
"It appears Mohammed Emwazi has been held to account for his callous and brutal crimes. However, it would have been far better for us all if he had been held to account in a court of law.
"These events only underline the necessity of accelerating international efforts, under the auspices of the UN, to bring an end to the Syrian conflict as part of a comprehensive regional settlement."
Mr Corbyn previously caused controversy by suggesting it was a "tragedy" that Osama bin Laden, the al-Qaeda leader, had been "assassinated" rather than being brought before the courts.
In response to Mr Corbyn's comments on Emwazi's death, one shadow minister said: "It is naive. It's not like arresting someone down a local shop.
"If you did try to arrest or capture him you would be putting a hell of a lot of people who would be put at risk. He has no comprehension."
Kevan Jones, the shadow defence minister, said that in an ideal world Emwazi would have been put on trial but "unfortunately we don't live in one". He added: "I think and most other people think a world without this individual is a better place."
Cage was criticised in February when Asim Qureshi, its research director, described Emwazi as a "beautiful young man" and the group claimed he may have turned to violence following harassment by the security services.
In the latest statement, Dr Adnan Siddiqui, director of Cage, said: "Emwazi's execution of defenceless hostages was inexcusable.
"But all avenues that led him to that point need to be investigated. Cage's repeated efforts and offers to negotiate for the release of Alan Henning were obstructed and squandered by the UK government and serious questions remain regarding these failures."