The entire world witnessed the Serbian atrocities against ethnic Albanians,
Before NATO's brutal bombing of Serbia, the only atrocities consisted of Kosovar Albanians committing terrorist attacks against Serbs, and frustrated Serbian police massacring villagers when they could not identify the terrorists who were hiding in those villages. It was quite a bit like frustrated US soldiers massacring Vietnamese villagers when they could not find the Vietcong who were hiding in a village.
I agree that those were atrocities, but those atrocities were being committed by both sides, the atrocities were being provoked by aggression from Kosovar Albanians, and Serbia's response was far from a systematic attack against Kosovar Albanians.
Serbia was agreeable to allowing NATO peacekeepers to help end the fighting, but NATO insisted that their forces be stationed permanently throughout all of Serbia. That was an unreasonable request, and Serbia was right to reject it.
NATO's bombing campaign to try to force Serbia to allow NATO forces to be permanently stationed throughout all of Serbia was an illegal act of aggression. And it ultimately didn't work. Serbia refused to sign until the provisions for permanently stationing NATO troops throughout Serbia were dropped.
Now, once NATO began their massive illegal bombardment of Serbia, then (and only then) an organized campaign of ethnic cleansing occurred against the Kosovar Albanians. But that was only a response to the massive act of aggression that was being waged towards Serbia.
More importantly, none of that justified violating Serbia's territorial integrity several years later and forcibly taking Kosovo away from them.
I agree with 100% of what you said about all of the horrible lies Russia always spouts about the nations they invade. But Russia still has a legitimate grievance regarding Serbia.
NATO did it to Serbia before Russia did it to Georgia and Ukraine. And now the West has little credibility when they argue that it's illegal for Russia to do it.