I'd think in this (and perhaps most) war(s) it comes down to the restraint, personal integrity or frustration of the individual soldier. Having just recently watched videos of (alleged) Russian saboteurs and infiltrators disguised in Ukrainian uniforms and GRADs shooting into residential areas, it appears Russia isn't above committing warcrimes even in these early stages of the invasion.

Not even speaking about sending in Chechen troops whose foremost reputation is perpetrating warcrime.