Thanos' Unoriginal Plan

In a book I'm reading currently, published in 1954, I suddenly came upon these lines of text on page 157:

"Oh, that." He seemed to come back from far away. "I don't know why he should be dangerous to me, but it's true that by all accounts he's a dangerous sort of chap."

"In what way?"

"Well, he's one of those half-balmy idealists who would quite happily kill off half of humanity if they thought for some reason it would be a good thing."

"I know the sort of person you mean."

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A truly serendipitous line of dialogue to come across in current year. I'm sure I've seen this plot-line/philosophy before in other villains/philosophies. I wonder what work first popularised the idea or whether it was some kind of common thought at some point.

While this isn't the plot of this book, they way they use the phrase so casually but also so specifically suggests to me that this premise goes way back.

Isn't that just an extreme case of "the ends justify the means"? Some form of utilitarism? In the end, if killing half of humanity is what it takes to make the rest of humanity happy (for the rest of it's existence), that may sound/seem like a comparitively small price to pay?

I guess that a similar line of thinking fits to many villians we have observed in history of humanity. I doubt that many of those considered themselves as evil, but probably had reasons that they themselves found to be convincing. It does not always take a sociopath to kill lots of people, given that there's a sufficient level of separation from the victims.

Indeed, but this notion of exactly half seems so specific. It's not 25% or 75% but specifically 50%. It sort of reeks of some specific line of thought. Real people in history tend to have all kinds of bizarre justifications for wholesale slaughter, but I don't think any of them have had the specific agenda of attempting a random 50%er. There must be some original meme somewhere IMO.