Until a few years ago, the answer would have been Assad ... he killed 300,000 of his own citizens in various ways, including gassing
But now, the "award" must go to Putin ... he's already sent 250,000 of his own troops to their deaths; when you combine that with foreign troops killed, plus innocent civilians - not just in the Ukrainian war, but also in the two other wars he started without any provocation - he easily exceeds Assad's number of victims, not even close.
Even China, Iran, and North Korea don't kill as many people as Putin.
That's why we call him a murderous monster!