Having searched for earlier versions of this video it appears it was first posted on the KatibatAhfadAlrasool Harem Youtube channel, titled "one of Assad's dogs executed in Harem". Harem is a small town in the Idlib region, located here, and the Ahfad al-Rasul [Grandchildren of the Prophet] brigade are believed to be a Jihadist/Salafist brigade operating outside of the FSA, who have been previously been involved with attacks on the Syrian military and security apparatus, including bombings in Damascus in co-operation with the Jihadist/Salafist group Ansar al-Islam
According to the description on the video, and discussion on various pro-opposition forums (for example here) the executed man was a officer in the Syrian Army (variously described as a sergeant major, sergeant, and lieutenant) who was apparently executed for writing pro-Assad slogans that transposed the word "God" with "Assad", which distorted a Koranic verse, committing heresy.
One translator I spoke to said he believed at one point in the video one of the men says "this is the dog who wrote [what sounds like] 'it all belongs to Bashar al-Assad'", and believes it might relate to a popular Jihadist verse "fight them until there is no more strife in the land, and faith belongs entirely to God", with "God" being replaced with "Assad". As of yet it's unclear if any other circumstances surround the execution, but it may relate to this video posted on the same day showing a large number of men arrested by Ahfad al-Rasul in the same region
Update December 31st Something else pointed out to me is that at the end of the video someone says "whoever shot from here, you hit the boy, whoever shot from here, you shot the boy, look!", and a poster on this forum berates the execution, stating a man nearby was shot accidentally during the execution.
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