It's what they do 'cause they think they can.
The goal also in their written and verbal statements on record is mass depopulation.
As if Ukraine/Russia had anything to do with Ukraine or Russia. The goal is to get rid of nearly everyone, since the 2020 thing failed to do the trick. Emptying out the nuke arsenals would do it, though (reading the least targeted areas and less likely to suffer radiation drift is an exercise for the reader, though a short one).