Most of the dead were legally citizens of the Russian Federation, but not what the leadership would really consider "Russian". To them, only people in the core Moscow - St. Petersburg region really count. Others are expendable. They may even see it as a net positive to purge some of population from the outer provinces: drunkards, unemployed, criminals, members of ethnic minorities, etc. (To be clear I don't support the Russian invasion of Ukraine, just explaining what Putin and his key supporters seem to be thinking.)
At WWII, Russian interrogated their own Russian villagers about what Germans did, and then murdered the interrogated Russian, for the chance that Germans later asked him what he told to the Russian army.
The large count of Russian deaths at WWII is in large part due to communists killing civilians and then blaming Germans or the war.
The thinking hasnt changed since Stalingrad, where civilians could've been evacuated and Stalin decided not to to help absorb the impact of the incoming army. Expendable.
It's just a shithole country.
Inb4 "Russia isn't the USSR" - yes it is.
Imagine if someone told Russians that somebody would kill half a million Russians soon. He would be considered the worst enemy of Russia. This enemy is Putin
Let's disregard that the data is unverified, and assume it is factual - that Russia has indeed lost around 400,000+ soldiers so far.
It really looks bad until you consider that:
1. The territories in East Ukraine - Donetsk (DPR), Luhansk (LPR), Zaporizhzhia and Kherson - that Russia seeks to annex had a population of around 10+ million in 2022 ( https://www.statista.com/statistics/1295222/ukraine-populati... ).
2. Conservative estimates of the population post-war, and under Russian occupation of Ukrainian territories is around 3.5 million ( https://frontliner.ua/en/nearly-a-fifth-of-the-country-life-... ).
3. Conservative estimate of the population in Crimea (which was annexed in 2014 and thus hasn't seen as much war displacement) is around 2.5 million.
So, if Russia manages to retain the territory it has occupied in Ukraine it is conservatively going to also add 6+ million people to its population. Even disregarding the economic benefits that Russia will accrue from the occupied territories, the cost-benefit analysis just in terms of population gain makes it clear why it is advantageous for the Russians to keep fighting, especially when it already has the advantage.