Self-defense. South Ossetia has been a de facto part of Russia, by its own volition and indeed its own initiative, since Independence. That Georgia's nominal pre-independence boundary was ever recognized internationally as its independent boundary is just a mistake due to lazyness.
Matters are infinitely murkier in Abkhazia, of course, as there, the strong anti-Georgian majority was engendered by (relatively nonfatal, not that that helps the Abkhazians' moral position particularly much) ethnic cleansing.
But on the issue of right and wrong in South Ossetia, there can't really be two opinions.
I also think that destroying the Georgian millitary potential, built for exactly such an invasion also was important.
The Russian government also needed to look strong before the various minorities, which weren't very happy with it... though the Ossetians are quite happy now.)
