Mussolini used poisoned gas in Ethiopia.
No doubt he was a war criminal and if his own people hadn’t shot him, he likely would have been hung by the allies. (With justification)
Still he wasn’t at the level of Hitler who intended to murder entire nations or Stalin who murdered 10’s of millions of people.
Where is Putin? Who knows, right now he is not Hitler, but if he starts a Nuclear war that kills billions of people then he will end up far worse.