What we do know is that the men, Cardiff-born Reyaad Khan, 21, and Ruhul Amin, from Aberdeen, reportedly planned attacks against and in the UK; plans for those attacks were uncovered by a Sky News undercover investigation.
Toward the end of their report on August 11 the Sky News investigator says "It's impossible to know if any of this is true, of course. But we suspect much of it is."
Does that sound a good enough reason to kill two British men in a country, Syria, which the UK is not at war with?
It comes after ex-top brass last night urged the PM to set a date for action.
War is a dirty business but there are rules of engagement. The Geneva Convention and Human Rights Acts around the world offer guidance and some protection; should we discard these rules so easily when it suits?
If we do we set dangerous precedents that can easily back fire.
Part of the problem is the alleged attacks planned against the UK were all scheduled for events that happened on or before the two men were killed-non took place.
Legal eagles in the UK have already said that our laws do not allow for retrospective action against possible attacks.
So many times when Cameron cannot act at will he tries to change or reform legislation. This time however it will not be easy. BBC News reports;
The "act of self defence" was lawful, despite MPs previously ruling out UK military action in Syria, the PM said.
Did others die in the 'precision airstrike'? Were any civilians caught up in the madness?
According to Forces online no civilians were killed and both men died in the same vehicle.
"The Prime Minister insisted that the strike was "entirely lawful" and was approved by Attorney General Jeremy Wright."
Attorney general Dominic Grieve was replaced by Jeremy Wright QC July 15, 2014. Wright has said the killing of Khan and Amin broke no rules but many people describe him as inexperienced and a 'yes' man.
One man that does have the necessary experience is Lord Dannatt who google describes as "a retired British Army officer and the incumbent Constable of the Tower of London. He was commissioned into the Green Howards in 1971, and his first tour of duty was in Belfast as a platoon commander."
Dannatt has previously worked for or with David Cameron as a defence adviser but he quit that role in 2010 when Cameron became UK PM.
And Lord Dannatt's conclusion is the "Strike On British IS Fighters "May Well Not Be Right"."
"I'm accepting that the British parliament has not authorised action from the air over Syria at the present time.
"Another British national, Junaid Hussain, 21 and from Birmingham, was killed in a separate air strike by US forces in Raqqa on August 24."
Note: Rupert Murdoch is the man behind Sky News and The Sun publication-"THE Sun today lays out the military options open to David Cameron for air strikes on Syria to ease the migrant crisis, was Sunday's headline. What a way to sell newspapers and war!
The UK would not hesitate to launch more secret drone strikes in Syria to thwart potential terror plots, Defence Secretary Michael Fallon has said reports BBC News-that presumably means illegal or not.
Today - David Cameron faces scrutiny over drone strikes against Britons in Syria
David Cameron knew RAF personnel were on bombing raids in Syria-July 2015
Cameron commits UK on US TV to war against Islamic State-also July 2015
British Prime Minister David Cameron loses vote on Syria action-August 2013