Will Russians actively come out into the streets and start killing Syrians? will they go that far to keep their base? they may have to.
Would we? Of course, if nothing else, then by covert operations. Russians have the same special forces we do.
They are probably already out there doing it than.
Count on it. Look at it from a Russian perspective. The fundamentalists topple Assad, then target the military base. The Russians will either have to start slaughtering on a wholesale basis or take a chance that weapons will go to... to.... you can guess. Fundamentalist muslims in Chechnya!