Shaikh Hasan Ba Shu’aib (may Allah preserve him) was asked:
Question: What is the Legislated ruling on the murder of the Russian ambassador in Turkey ? And what is the ruling on killing this ambassador as form of support for the Muslims in Syria?
Answer: “He [takes] the ruling of a disbeliever who [signed] a covenant and one who is given or has sought safety. So murdering him is considered a betrayal of the covenant due to the statement of our Prophet (may peace and blessings be upon him):
‎ من قتل نفسًا معاهدًا لم يُرِحْ رائحة الجنة
Whoever kills a Mu’aahad (one who has taken a covenant) that he will not smell the scent of Paradise
The issue is one of religion and not empty zeal/enthusiasm or a violent storm of emotion” 
Translated by
Faisal Ibn Abdul Qaadir Ibn Hassan

Abu Sulaymaan