The blood of a Muslim man who testifies that there is no god but Allah and that I am the Messenger of Allah should not be lawfully shed but only for one of three reasons: married fornicator, soul for soul, and one who deserts his religion separating himself from the community. (Using translation from Abū Dāʾūd 4352)   

قَالَ رَسُولُ اللَّهِ ﷺ لَا يَحِلُّ دَمُ رَجُلٍ يَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللَّهُ إِلَّا إِحْدَى ثَلَاثَةِ نَفَرٍ النَّفْسُ بِالنَّفْسِ وَالثَّيِّبُ الزَّانِي وَالتَّارِكُ لِدِينِهِ الْمُفَارِقُ لِلْجَمَاعَةِ