I don’t doubt that people on both ends of the political spectrum want to disavow a view they see as obviously repugnant and blame it on their opponents. However, it also seems clear to me that the grounds on which people on the Left call Thanos’s worldview pro-capitalist and people on the Right call it socialist are quite different. In short, the leftist critics on Tumblr attribute his factual assumptions to capitalist ideology, while your Rand-supporting father is attributing his normative framework to a socialist mindset.

What people on Tumblr have been saying in various ways is that “overpopulation is a myth”: it is a false causal explanation for the existence and persistence of material scarcity. The idea of overpopulation, this criticism goes, enables rich people to blame poor people for their own poverty. The problem, these rich capitalists say, isn’t the distribution of resources; it isn’t that rich countries overproduce food and throw most of it away, or that rich people hoard money, or that powerful corporations renew patents on life-saving technology to make sure that the products remain scarce and expensive. It’s that those stupid poor people with no self-control just won’t stop making babies. I suspect that this criticism is something of an oversimplification; a growing human population will have more and more energy demands, which may or may not be possible to meet with only renewable energy resources, and will of course require more and more food, which will in turn require that wilderness be cleared for agriculture (unless urban farming and vertical gardens really catch on). On the other hand, population growth rates do slow down as societies become more educated and gender-egalitarian. So I suspect that the gap between the West and the rest of the world (whose labor and resources the West has been exploiting for the past few centuries) is a large part of the apparent problem, and if that gap were allowed to close, there would no longer be any reason to worry about runaway population growth.

What your father seems to be voicing is a general criticism of consequentialist ethics (which operates by maximizing some good outcome): that it aggregates well-being, and therefore has no problem sacrificing the well-being of a few people in order to improve the situation of a large number of people. The most common form of consequentialism is utilitarianism, for which the good to be maximized is pleasure or happiness. A criticism originally voiced by John Rawls and taken up by various other philosophical critics of utilitarianism is that it fails to recognize or respect “the separateness of persons.” It’s perfectly fine for one person to forgo a benefit at one time in order to enjoy a greater benefit at a later time, e.g., by saving and investing money, or by refraining from indulgences in order to preserve one’s health, because the near-term costs are borne by the same person who enjoys the long-term benefits. Utilitarianism makes the same kind of calculation across populations rather than across time, imposing smaller costs in one place in order to reap greater benefits elsewhere; but this is illegitimate (the criticism goes) because the people who bear the costs are not the same as the people who enjoy the benefits. Pretty obviously Thanos is reasoning in a consequentialist/utilitarian way: he’s trying to maximize average happiness by replacing a large number of low-quality lives with a smaller number of high-quality lives. It sucks for the people who die (or maybe not, since he wants to give them a quick, painless death) and for the people who lose loved ones, but in theory, things will be a lot better for the next few generations.

Consequentialism is a kind of collectivist thinking, you might say: the goal is to maximize well-being (however that’s defined) across the entire population of moral patients (creatures who deserve moral consideration, which might be humans, intelligent beings, sentient beings, all living beings…) without regard for how that well-being is distributed among the individuals. As a practical matter, utilitarianism tends to promote egalitarian distribution of resources because of the phenomenon of diminishing marginal utility: each added unit of whatever goods (money, food, etc.) provides more pleasure/happiness to someone with less of it than to someone with more of it, so you’ll tend to maximize happiness as the distribution nears equality – ignoring things like different individuals’ utility functions (i.e., how much pleasure/happiness each person gets from one unit of the good at each level of prior possession). So I can see how someone might think of utilitarianism as a “socialist” style of ethics… except that a lot of socialists hate it, too, and attribute it to capitalism (Bentham’s utilitarianism is one of the foundations of modern economic theory). But then both socialists and capitalists like to accuse each other of regarding human life as expendable, as something you can put a numerical value on, rather than as sacred, inviolable, possessing immeasurable dignity rather than a measurable price.