Resources and energy and the ability to meet basic human needs are, in fact, almost entirely NOT zero sum.
Virtually every resource we have, whether oil, minerals, food, shelter, entertainment, education, etc, is drastically expanded and in many cases is only possible by living in a society with many other people. There's vastly more oil, copper, food, shelter, entertainment, and education now than at any other time in history. That's because human ingenuity and economies of scale are fundamentally positive-sum.
Imagine you killed half of all people. Will your lot improve?
Let's say 99.9% of people are killed. I am certain that your life would rapidly become extremely difficult and would remain that way for the rest of your life.
There can be no industrial civilization without economies of scale. To pick a very Hacker News example, think about Moore's Law. It has only been able to be continued because the market for computing has continually expanded. If the revenue remains flat for semiconductors, there's not enough capital to be able to invest in the next generation of equipment that is yet more expensive. You need more people buying these chips in order to justify that greater investment.