One can read more here. Do the following hadiths justify these executions?
1) Abu Ya’la narrated in his Musnad from Ibn ‘Abbaas (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “The bonds of Islam and the bases of religion are three, on which Islam was founded. Whoever gives up one of them becomes a kafir thereby and it is permissible to shed his blood: the testimony that there is no god but Allaah, the prescribed prayers and fasting Ramadan.”
2} Narrated Ikrima, "Some atheists were brought to Ali and he burnt them. The news of this event, reached Ibn Abbas who said, "If I had been in his place, I would not have burnt them, as Allah's messenger forbade it, saying, "Do not punish anybody with Allah's punishment (fire)." I would have killed them according to the statement of Allah's Messenger, "Whoever changed his Islamic religion, then kill him." [Sahih Bukhari 9:57]
1) Abu Ya’la narrated in his Musnad from Ibn ‘Abbaas (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “The bonds of Islam and the bases of religion are three, on which Islam was founded. Whoever gives up one of them becomes a kafir thereby and it is permissible to shed his blood: the testimony that there is no god but Allaah, the prescribed prayers and fasting Ramadan.”
2} Narrated Ikrima, "Some atheists were brought to Ali and he burnt them. The news of this event, reached Ibn Abbas who said, "If I had been in his place, I would not have burnt them, as Allah's messenger forbade it, saying, "Do not punish anybody with Allah's punishment (fire)." I would have killed them according to the statement of Allah's Messenger, "Whoever changed his Islamic religion, then kill him." [Sahih Bukhari 9:57]