Turkey is largely made up of Sunni Muslims. Turkey, Russia, and Iran are in Syria slaughtering Sunni Muslims. This is why the assassin did what he did on behalf of his countrymen who are perplexed and distraught at what they see their own country is doing in Syria.
Not rationalizing, just trying to shed some light on a very confusing set of circumstances.
I see it differently. The Sunnis ( including ISIS) are in Syria to overthrow the sovereign and elected government of Bashar Al Assad through terrorism and insurrection. Assad is of the Alawite sect and sympathetic to the Shiites . The Sunnis took over in Aleppo within the sovereign country of Syria . Syria ,under international law, requested assistance from the Russian to push out the Sunni terrorist. That was granted . Now the Sunni bastards are getting slaughtered and have lost Aleppo real bad, The next little party will be a Raqqa , a city known as the Isis HQ in the illegally held sovereign territory of Syria. We can now see President Trump , at the legal request of Assad, committing ground and air power along with the Russian invitees to kill thousands of those gawdless bastards real dead and remove them from Syria 100%! McCain can go to hell.