European Council for Fatwa and Research: "The Apostate is Killed in Order to Protect the Religion and the Community"
Translated from the Muslim Brotherhood-linked European Council for Fatwa and Research. The justification here is that the apostate from Islam is killed not because of his belief, but because he essentially commits high treason by coming out publicly against Islam, i.e. the state (the same argument is made here and here). However, comparing apostasy in Islam with committing high treason is a false equivalence. A more accurate analogy would be that of apostasy and renouncing your citizenship. Unsurprisingly, there is no death penalty for the latter.
A: The issue of killing the apostate is a function of the state. His judgment belongs with the Islamic government. This is not the concern of Islamic foundations, associations, or centers. A group of Salafis and Imams are of the opinion that not every apostate should be killed, but rather only those who openly commit apostasy, or call for fitna, or voice harmful things against Allah and His Prophet (peace be upon him) and the believers. [The apostate] is killed in order to protect the religion and the community from his corruption, and not to restrict freedoms, as he by his action is infringing on the rights of others. The interests of the state and the society come before individual self-interest. In truth, this issue is similar to what is termed in contemporary law as “high treason” because of the harm to the public that it causes.
European Council for Fatwa and Research, August 16, 2008