Georgia claims they have suffered 129 casualties, mostly soldiers. Russia and South Ossetia claim ~1500 to 2000 deaths, all civilians, at the hands of Georgian troops. Please explain why you label this is a "brutal and disproportionate" response. On the scale of things this seems quite a measured response. Can you name another military action to drive an invader from an urban area that resulted in less than 60 civilian casualties?
While we can all speculate on how impure Russia's motives may be this seems like as small a body count as could practically be expected when driving an aggressor invader out. While we can also all loath military actions that kill civilians there are times when an aggressor must be stopped immediately, such as when the cost in lives of doing so will be less than not doing so. When this is the case every care must be taken to use the minimum force necessary to accomplish this goal. This seems like a near textbook case of this belief in action, regardless of any additional motives Russian leaders may have.
John Thornton