The destruction in Syria is not just down to Russia but... Russia is responsible for a lot of the destruction there. The total war style tactics used in Syria were brutal, even compared to normal modern warfare which is inherently brutal.
The longer this war goes on, the more dark it will get. Russia's shown domestically in Chechnya and abroad in Syria that they don't care if they have to completely destroy cities in order to achieve their military goals.
The Azov Regiment defending Mariupol are now saying Russia's used chemical weapons. It's hard to really believe that as the Azov Regiment are, unfortunately, neo-Nazi dickheads and they're making this claim of a red line being crossed while there are no reporters left in the besieged city to verify the claims. So that could just be a group of unscrupulous Nazis trying to pull NATO into the war.
But it's definitely concerning that the man in charge of Syria is now in charge of Ukraine. He does not give a shit about civilian deaths, collateral damage, or war crimes.