A summary of Sunni law on apostasy given by Mohammed Al Abdari Ibn Hadj in Al Madkhal quoted by S. M. Zwemer in The Law of Apostasy in Islam Quotes

As for apostates, it is permitted to kill them by facing them or coming upon them from behind… Secondly, their blood if shed brings no vengeance. Thirdly, their property is the spoil of true believers. Fourthly, their marriage ties become null and void.