The Assassination of Dimitri was a minor conflict in Syphon Filter: The Omega Strain in which Gary Stoneman was sent to stop the Italian mafia from using the virus in their fight against rival gangsters.
The confrontation was short but significant, and involved Stone sniping the honcho, Dimitri Alexopolous, before the latter could acquire Syphon Filter. Left without a head, the mafia was leaderless temporarily, but that was enough for Stone to focus on other threats while they attempted to search for a second boss.