I think Thanos has a strong pragmatic streak along with his powerful idealism. And, in his defense, in the time it takes the populations to recover, their associated ecosystems will recover significantly as well. The Black Death did largely solve the major problems plaguing Europe before it struck (pardon the pun).
Slightly off topic, my brother and I were discussing the Black Death's role in history yesterday in a conversation inspired by this post, and we decided that if it had never struck Europe, Europe would have remained a backwater and we wouldn't live in a Western dominated world today. (Mix that with the Chinese not cancelling the treasure fleets, and there's a pretty cool alternate history. Hmmm. Might need to write that.)
I think Thanos was not a psychopath, or rather has not degenerated into a psychopathic monster as many granted absolute power tends to become. Could it be that he had doubts regarding his ideologic infallibility? Or maybe he was an opportunist, but what did Thanos desire to gain, when he already possessed awesome power? Could it be that he was, indeed, incompetent?
I wonder if the West would have developed their emphasis on technical/mechnical aptitude without the Black Death. When 33% of the work force lies in shallow, mass graves, the value of workers tend to rise exponentially. The counter-weight crane and trebuchet, so valued in the West, was but an insignificant curiosity in China because of the over-abundance of human muscle available.
I would think that Europe would develop along a similar technical trajectory of using manpower over mechanical assistance. I wonder how Christian theology would have developed in a world that continued to relegate much of the population as mechanical labor force. Would Europe have been able to construct those magnificent stone Gothic cathedrals, or would Europe be content to manufacture either wooden structures or small scale stone churches? Would feudal and mercenary military matrix have evolved into mass conscription earlier, due to mass availability of cheap manpower? Could there have been a Reformation or the Revolution?
Europe absent the Black Death would be a fantastic story! It would involve evaluation of sociocultural, sociopolitical, and sociotechnical perspectives. We need to find a publisher to get your project started!