Sanctity of a Muslim's life
أُمِرْتُ أَنْ أُقَاتِلَ النَّاسَ حَتَّى يَشْهَدُوا أَنْ لَا إِلَهَ إِلَّا اللَّهُ وَأَنَّ مُحَمَّدًا رَسُولُ اللَّهِ
Umirtu an uqatila-n-nasa hatta yashhadu an la ilaha illa-llah wa anna Muhammadan rasulullah.
"I have been ordered to fight people until they testify that there is no god but Allah and that Muhammad is the Messenger of Allah, if they do that, their lives and property are protected from me except by right."
Narrated by Ibn Umar. Sahih al-Bukhari 25, Sahih Muslim 22 · Nawawi's Forty · #8
A short reflection
The shahadah is the threshold. Once it's declared, a Muslim's life and property are sacred.