If he had been produced in front of a Qadhi, would this interpretation (it is a black law with the meaning that the law is good but its execution is faulty and contrary to Islam) been something that could have been presented as a defense against the death penalty being handed down on him? This is one issue that has to be considered. Another one is that we have people in the Muslim-majority countries (including Pakistan) who rally in the streets saying that this or that aspect of Shariah is wrong and they want no part of it in the land they are living in. From what I understand, this is open apostasy, and if it is correct that Taseer was to be killed, so is killing such types of people also allowed (that is, if the country is actually Islamic, as killing the murtad can be done by anyone, even though it is wrong from the point of view of overstepping the rights of the Islamic ruler. The fact that such people can rally and gather millions of protesters and votes against Shariah means that something more fundamental is wrong in the fabric of the society, but that is another matter).