Thanos’ ideas about everything were extremely silly. A halving of population would do nothing on any time scale. A small blip easily recovered by reproduction. People deserve better writing, even in comic book movies.
It's outrageously comic-book-y, but may as well lean into the illogic.
They even teased it at the end of The Avengers: the Chitauri leader tells Thanos "to challenge [humans] is to court death", at which time Thanos grins eagerly. But with Earth-199999's Thanos dead, they're going to have to invoke some multiversal malarkey if they want to develop that storyline now.
See: Pol Pot, Adolf Hitler, ISIS, Kim Jong Un, and numerous other examples.
If they were rational they would not be despots and fanatics. Unfortunately charismatic lunatics command followings. Charisma bypasses the rational mind and shoots straight into the limbic system (as do fear, outrage, etc).
Cult leaders are good cases too: L. Ron Hubbard, Jim Jones, Charles Manson, Keith Raniere, etc. These people would be as bad as the worst despots if they actually had power over more than a few people.
Thanos' line is more rational than this: https://en.wikipedia.org/wiki/Xenu
The population was half of 2020 in 1973 (1). Emissions were also half of now back then (2). That’s nearly 30 years of runway that Thano’s solution would provide. With today’s technology and rapid iteration, we could definitely do something better for climate change.
I’m not endorsing the method, but merely disagreeing with your assertion.
(1) https://www.worldometers.info/world-population/world-populat...
(2) https://www.statista.com/statistics/276629/global-co2-emissi...