Worth noting that the incident to which you are referring was a case of non-military Russians employed by Assad as mercenaries, who attempted to attack US forces in Syria. The Russian military and the US military in Syria are in fairly close communication, and both are taking pains not to strike each other. The Russian government likely does not care much about the rogue mercenaries who made a dumb decision to attack Americans, and certainly are not going to retaliate by attacking other Americans in other countries. Considering that the attack on the Americans is the kind of thing that could kickstart a war, I’d almost guess that the Russian government would retaliate against the private security contractor for nearly destabilizing the situation and almost provoking a US counter attack against Russian military forces.