Killing of Ambassadors is Forbidden in Islam

After the news of the death of the Russian Ambassador of Turkey comes, it's imperative for us to understand the following:

Two messengers (ambassadors) came to Prophet Muhammad (S) from the followers of Musaylamah, a false prophet who later rebelled and killed many Muslims. Though their speech angered the Prophet (S), he let them leave in peace, saying, "...The messengers are not to be killed..." (Abu Dawood)

Prophet Muhammad (S) said "Whoever kills a mu'ahid (a non-Muslim given protection in a Muslim land), he will never smell the fragrance of Paradise." [Bukhari]

The scholars defined "mu'ahid" as a non-Muslim whose country has a treaty of peace (even if temporary) with a Muslim country, or a non-Muslim from a country at war with Muslims who is given protection during his stay in the Muslim country. His life, property, and family is protected - his blood is not to be spilled. The one who kills the mu'ahid, his abode is the Hellfire.

Scholars have deemed the life of ambassadors, diplomats, and messengers as protected, even if they are from a nation the Muslims are at war with.

“It became the Sunnah of the Prophet that a Messenger must not be killed" - Ibn Qayyim

“People who come from states which are at war with us who are given a covenant of peace cannot be killed, nor can their lives or property be transgressed against.” -Ibn Qudamah

"If agreements are made with the messenger (Ambassador, Diplomat), Muslims must respect them and must not betray the messengers of their enemies, even if the enemy has killed Muslim prisoners. And their messengers should not be killed in revenge” -Abu Yusuf, student of Abu Hanifa