Whoever carried out the order to breach the Yellow River dams in 1938 directly caused the death of 800,000 people (an estimate; we'll never know the true amount, but the lowest estimate is half a million) plus untold environmental damage, homelessness, and displacement.

The order was given by Chiang Kai-shek (during WWII, the Chinese Nationalist government thought that a big flood would stop the Japanese advance), but you're not looking for the commanders. In this case, the order would have been carried out by a smallish group; I can't think of anyone else whose direct, personal actions killed so many.

I have never come across any information on who actually carried out the order or how they coped with their action.

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