The Russians don't understand why is everyone making a big deal out of the fact that they are providing sophisticated weapons to the brutal Assad regime. The problem for Israel is not the Assad regime, as much as they would like to look sympathetic to the plight of the poor victims of the civil war with Syria, the real danger to Israel is the transfer of these weapons to the Hezbollah. These weapons include Scud - D surface to surface missiles, SA-17 and S-300 air defense systems and Surface to Sea Yahunt missiles.
The meetings that the Israeli prime minister just had in Russia were not pleasant; The Russians basically told him not to try and meddle in their business and in return the defiant prime minister threatened he will attack every shipment of weapons he could identify. In response, the Russians sent 4 new war crafts to the area increasing the level of possibility of a confrontation. Will that ever happen? Its hard to believe but its possible.
The Syrians don't know how to operate these advanced weapons systems and therefore they acquire the services of Russian consultants an sometimes military personal. In case Israel decides to attack one of the shipments, its possible that Russian personnel will be hurt by that action (Iranian personnel did die in the past Israeli attacks on weapons shipments; see my previous post on that).
Will the Russians retaliate directly against Israeli targets? If they do, it would be the first time a direct confrontation has happened between one of the superpowers and Israel.
Difficult to imagine but not incomprehensible.