Igor Sergun, Former Director of GRU, the Main Intelligence Directorate of the General Staff of the Armed Forces of the Russian Federation
The anonymous source starts off his rant with:
In other words no sooner than he opens his mouth it becomes a guessing game: "Could", "May", "Must have".[I cannot] "rule out that his death could have been the result of a complicated intelligence security operation in which several Arab and Middle Eastern intelligence actors may have participated. Moscow must have discovered some clues to this matter."
Evidence that Sergun was murdered? None whatsoever is offered. That doesn't stop preclude even more dubious claims. The believability is pushed to the limit in saying that Sergun's death is the cause of the tensions between Turkey and Russia, implicating specifically the MIT (Turkey Secret Service) as having carried out the hit on the former GRU commander
Like for MH17, Chemical attacks in Syria, Iraq's WMD and many other issues that have shaped the course of the world in the last years, no evidence is provided to sustain the accusations against Russia or its allies. Anonymous sources are compared to hard rock-solid evidence, opinions become best-practice and hopes and wishes are used to fuel american exceptionalism and White house foreign policy strategies."It is this that made Russia's decision for confrontation with Turkey decisive and final."
It does not matter what really happened, it's just important to blame it on Putin no matter how anonymous, wild or ridicules the accusations look.
Comment: The cause of tensions between Russia and Turkey could more realistically be blamed on Turkey's shooting down Russian aircraft, mocking their grief, and then repeatedly bombing Russia's allies.