There are many people in history who have, directly or indirectly, been responsible for massive amounts of deaths. Dictators, cult leaders, military leaders... you name it. But who is the person who is responsible for the largest amount of deaths?
Maybe Hitler, who is pretty directly responsible of not only the six million jews killed in the holocaust, but arguably also the 60 or so million people killed in World War II? Perhaps Joseph Stalin, whose extermination of people in Gulag camps and otherwise simply cannot be counted because not everything is recorded, but rivals those of Hitler?
Some have argued for a rather different kind of person, for a rather different kind of reason: Their candidate is Thomas Midgley Jr, an American engineer and chemist born in 1889.
How can an American engineer be responsible for even more deaths than Hitler himself?
This is the single person that can be attributed to have caused the most serious and longest-lasting environmental damage by humanity (or by any living organism for that matter) in its entire history.
He was the lead chemist in the team that discovered that adding a lead compound to gasoline improved the efficiency of internal combustion engines. Moreover, he deliberately used misleading marketing to hide the fact that the compound contained lead (lead poisoning was already a well-known phenomenon back in those days, and hiding the fact that it contained copious amounts of lead was completely deliberate.)
That happened in the 1920's. Lead is still used in gasoline in many countries even today, almost a hundred years later, regardless of the known environmental impact it has. The amount of lead released to the environment during these hundred years is simply staggering. A person simply cannot comprehend the sheer amount of it that has been dumped onto the environment this way.
But that's not all. In a coincidence that defies belief, the exact same Thomas Midgley Jr. was the inventor of using freon gases in refrigeration and gas propellants. Yes, those freon gases that are some of the most ozone-depleting substances ever created by humanity.
Thomas Midgley Jr. is arguably the single person who is the responsible for the most environmental damage ever created by humanity. Environmental damage that is still going on today, almost a hundred years later, and will persist for probably hundreds of years to come.
Environmental damage that has caused uncountable amount of illness and deaths. It's impossible to even estimate exact numbers, but certainly more than even the deaths of World War II.
Sometimes you can be responsible for the deaths of tens and even hundreds of millions of people, without even realizing it.
It wouldn't be so bad if he simply had always had good intentions, and it was all just a big accident. But he knew of the problems of adding lead to gasoline, he knew of the problems of lead poisoning (something that he himself had suffered from), yet he deliberately used misleading marketing to hide the fact. That makes him quite directly responsible.
To be fair, he did not know of the effects of freon gas on the environment. That doesn't diminish the impact, though.