I don't know who that might be, but the person that contributed the most to the rise of factory farming might actually have done even more direct harm than Hitler/Mao/Stalin, maybe even them combined?

Even if we can't figure out the exact number 1 contributor in factory farming, some of their big players can also easily surpass Hitler/Mao/Stalin. Frank Purdue, the founder of Purdue, is likely a top 100 contributor to factory farming as his company now kills 700 million chickens a year, and some of the standard methods in the chicken raising industry was rumored to be invented by him. Of course, we can't attribute all these to him 100%, but it sounds reasonable to claim that he contributed at least 10% of those numbers.

Some Chinese leaders might have been bigger contributors than Purdue, for China is quickly becoming the biggest factory farming country (in terms of the number of animals it already is). And Chinese leaders played huge roles in deciding China to popularize factory farming. 

And then maybe factory farming isn't the worse. Besides passively ignoring wild animals' suffering, humans also cause them harm actively. So it could be the case that an authoritarian ruler of a big country (which could have been Hitler/Mao/Stalin) could have caused more harm than Purdue or whoever is the no.1 contributor to factory farming. For example, city constructions cause a lot of animals to die. And if you want a more vividly harmful example, during Mao's Great Leap Forward, there was a "kill the four pests campaign", giving prizes to people who kill and present the animals. It caused a lot of people to become professional hunters, and some of them also decided to just raise the animals and then turn them into corpses for the prizes. One estimate thinks that "the government and the public were responsible for the deaths of 1.5 billion rats, 1 billion sparrows, over 220 million pounds of flies, and over 24 million pounds of mosquitoes". And it's only one of Mao's animal-harming projects. (a theory also says the great famine was partially caused by this.)

Some obvious answers include:

1. Columbus - for initiating the colonization of the Americas which cost tens of millions of lives, and as a result also the transatlantic slave tried which cost millions more.
2. Mao - for his policies that caused tens of millions to die of hunger, and for creating an enduring totalitarian regime over what is today 1.4 billion people.
3. Stalin - for similarly killing tens of millions with his policies, some of them directly by murder.
4. Hitler - for directly and purposefully murdering millions, and for starting the war that killed tens of millions in addition.

• Fritz Haber? Responsible for both chemical weapons and synthetic fertilizer. I'm not sure.

The first two are a mix between direct harms and altering the course of history, while the second two are mostly about direct harms. Haber did much direct harm, but the evaluation of the change caused by his synthesis of ammonia is more complicated.

I don't have time to answer the bonus questions, but am interested in such answers.

I asked GPT-3 your question 10 times. Answers: 
- Hitler 7

- Judas Iscariot 1

- Napolean Bonaparte 1

- Genghis Khan 1

I then tried to exclude Hitler by saying "Aside from Adolf Hitler" and asked this 10 times as well (some answers gave multiple people). Answers: 

- Stalin 5

- Mao Zedong 3

- Pol Pot 2

- Christopher Columbus 1

- Bashar al-Assad 1

The answer to the bonus questions is basically always of the form: "The obvious counterfactual to this harm is that Stalin never came to power, or that he was removed from power before he could do any damage. The ideal counterfactual is that Stalin never existed. As for what an ambitious, altruistic, and talented person at the time could have done to mitigate this harm, it is difficult to say. More hypothetically, an EA-like community could have worked to remove Stalin from power, or to prevent him from ever coming to power in the first place."

Not sure how helpful this is, but perhaps it is interesting to get a sense of what the "typical" answer might be.