How do you know?
Do you get your fake news from Russian news media, sorry, propaganda media.
The Kremlin has a long history of lies.
The Russian side has camera teams in Mariupol, other international media being there as well. They filmed and made interviews. None of the locals was happy about the situation, some even described how relatives died and how dire their situation is or was. But its all because of the ongoing fights with the Ukrainian nationalists in the city. To describe it as a war against the civilian population is just fake propaganda. The Russians could care more, yes, and the war as such is horrible, but there is no need to invent a "genocidal" narrative, other than causing a propaganda war.
The Western media selectively report only the Ukrainian propaganda news, if the Russian side reports atrocities or films whitnesses, they being not even checked, let alone reported. Many of the "news" the Western media publish come direct from the CIA and the SBU. I listen to both sides and neutral reports from e.g. Indian news agencies. That's all.