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.