Russian Army? Nothing. The police force would be likely to cordon off areas, and a special ops group or three would be dispatched to monitor the situation and prevent escalation, but honestly, I think that would be it. Contrary to your image of the ideal patriotic soldier, most of our armed forces - and, evidently, your armed forces - seem to be at least decent enough human beings that discharging high-caliber automatic firearms into a crowd of unarmed anyone-s is pretty close to the bottom on their scale of acceptable responses.
Haven't the Russian security people solved several such problems with Chechnyans using the more dakka approach? An incident where they gassed hostages and terrorists alike and killed a bunch of hostages in a theatre springs to mind.
Yeah, that one's pretty notorious. The plan was sound, the problem was that there were no medical teams on standby. Many unconscious people suffocated due to the haste with which the building had to be cleared - they were in some cases literally piled up as they were taken out of the theater. So yeah, they are not without their failings, but they are not cruel enough to knowingly kill civilians - the whole idea of that plan was to take everyone out, so that civilians could not be used as shields. The russian phrase that describes it best is "Хотели как лучше, а получилось как всегда" (lit. "wanted to do what's better, ended up doing as usual" - a good plan ending up going badly).
Yes, that one incident was a brutal fuck-up, but it was a fuck-up nevertheless, not a deliberate killing of hostages.
As for the soldiers in Chechnya, they wanted to gun down the locals there little more than the American soldiers wanted to gun down the locals in Vietnam and Iraq, but ended up doing that anyway because they were ordered to by the Soviet-style kill-happy government, and then mutual killings started and mutual hate sprung up.
In general, when talking about Ukrainian and Russian soldiers, I would like to refer you to this excellent quote by GlyphGryph:
There are plenty of decent people in our government, but a small group of power-hogging tyrants has all the power, yes.
The decent people that successfully remain in government tend to be those of the sort that don't rock the boat, for obvious reasons. Even the soviets and fascists of World War II counted many decent people among their ranks. Sometimes you're choice is to do what good you can within a broken system, to do nothing, or to be destroyed for opposing it.
That's true the world over, though - many terrible organizations, many organizations run by selfish, greedy, corrupt individuals, function only because of the decent individuals who hold them up, who often would rather not but have no realistic choice when they have people depending upon them and where refusal does not disrupt the system but only disempowers the individual.
This isn't really a commentary on on Russia in the slightest, just on the idea that a bad organization run by bad people doesn't necessarily mean it's made of said bad people, and that it usually isn't.