Qur'an (4:89) - "They but wish that ye should reject Faith, as they do, and thus be on the same footing (as they): But take not friends from their ranks until they flee in the way of Allah (From what is forbidden). But if they turn renegades, seize them and slay them wherever ye find them"
Council of ExMuslim
Qur'an (9:11-12) - "But if they repent and establish worship and pay the poor-due, then are they your brethren in religion. We detail Our revelations for a people who have knowledge. And if they break their pledges after their treaty (hath been made with you) and assail your religion, then fight the heads of disbelief - Lo! they have no binding oaths - in order that they may desist." This verse is speaking of infidels (ie. "slay the infidels wherever you find them" 9:5) who obviously became Muslim to escape the sword, but the Hadith make no distinction of how a Muslim came to be a Muslim. Apostasy is always punished by death.
Other verses that seem to support the many Hadith demanding death for apostates are Qur'an verses 2:217, 9:73-74, 88:21, 5:54, and 9:66.
Advocates for killing apostates have pointed out that the supporting hadith are reliable and thus qualify as law according to verse 4:80 - "Whoso obeyeth the Messenger obeyeth Allah."
From the Hadith:
The reason why executing apostates has always been well-ensconced in Islamic law is that there is an indisputable record of Muhammad and his companions doing exactly that.
Bukhari (52:260) - "...The Prophet said, 'If somebody (a Muslim) discards his religion, kill him.' "
Bukhari (83:37) - "Allah's Apostle never killed anyone except in one of the following three situations: (1) A person who killed somebody unjustly, was killed (in Qisas,) (2) a married person who committed illegal sexual intercourse and (3) a man who fought against Allah and His Apostle and deserted Islam and became an apostate."
Bukhari (84:57) - "[In the words of] Allah's Apostle, 'Whoever changed his Islamic religion, then kill him.'"
Bukhari (89:271) - A man who embraces Islam, then reverts to Judaism is to be killed according to "the verdict of Allah and his apostle."