Fighters of Al-Qaeda front group the Islamic State of Iraq and Syria accused Mohammad Qattaa of blasphemy but a human rights group said he had done nothing more than use a common Arabic phrase that makes reference to the Prophet Mohammed's name.
Pictures of Qattaa's face, bloodied by the three shots that cut short his life on Sunday, spread like wildfire on social media websites, prompting a strong condemnation of his killing from the mainstream opposition.
"Where are his rights? He was a child! How could they kill him?
"They killed him right in front of my eyes... May God take revenge on them... I saw his blood streaming down," she wailed.
"We are with neither side (in Syria's raging conflict). We just look after ourselves... Why did you kill my son? Is he a terrorist?"
The Syrian Observatory for Human Rights said the militants responsible for the boy's execution were foreign volunteers with the Al-Qaeda-linked Islamic State of Iraq and Syria.
"Those who executed the boy were not Syrian," he added.
Large swathes of northern Syria have fallen into the hands of various rebel groups, some of them Islamist, which have set up their own justice systems in areas they control.
Qattaa's killing was the latest in a series of abuses by rebel fighters that have discomfited the mainstream opposition and its Western backers.
Just last month, amateur video showed a rebel fighter in the devastated province of Homs eating the organs of a slain soldier.
"As the conflict develops, it is logical that we will see more problems of criminality emerge," said Aron Lund, who has written extensively on the Syrian insurgency.
