Many would say the regime could not do the massacre of Karm Zaitoun. they would argue that it can serve no purpose for the regime especially on the day the NSC is gathering to discuss situation in Syria.
From what i see first the regime do not care what is happening outside of Syria, they never did. Few months ago FM Moallem apologized for attacking foreign embassies, but on same day regime thugs attacked two more embassies in the afternoon on the same day. The regime bombed Baba Amro while regional media is airing the attack live on TV, and international media reporters reported events daily for 27 days. But second and more important reason, is this is the whole point. The regime cannot keep the army in front each house in the country, they need people to fear the army and the regime. So they need to keep population submission through fear tactics. this methods have been tested and used with great success in the 80s in Hama. They take an area or a town and hit it so hard and so brutal to use it as an example for the rest. So shelling Baba Amro randomly until leaving no building intact, and killing brutally 50 people in karm zaitoun while allowing media pictures to escape is the intended action and the goal. and finally we know from history and experience the regime is capable of doing such and event while I do not know if opposition can do something horrible like that.
Media will give different versions for what actually happened and by whom, pro regime media will accuse opposition and outside media will accuse regime. I am currently outside of Syria so i will not know what the truth actually is, but in my logic i look at the people on the ground if they blame regime thugs and opposition members. from what i see for now; i see people are fleeing areas the Syrian army is entering and still providing cover and support for opposition fighters, so I blame the regime.