I think people are overlooking the emotional aspect to Thanos, being that he lost his home world. In that instance he had a plan, a fairly crazy plan to kill half the population... but one that oddly would have saved lives in that instance versus the alternative. As a result, people he would have been personally familiar with died as he saw his plan overlooked... I can only imagine what it would be like living through a world that is starving and the apocalypse-like scenario that transpired that caused that world to die.
That experience made Thanos obsessive and misguided, to the point he sees himself the savior by transplanting his idea on a universe scale. Similarly, he has become so obsessed he is able to shut out serious critiques to his plan, likely bore from feeling it was people opposed to his original plan on Titan that were eventually the problem that led to the planet dying.
We got none of that. What we got was Thanos rambling and a screen shot from a completely untrustworthy narrator.
It was clearly an FTL capable civilization that should have been or near post scarcity. Whatever killed the planet couldn't have been artificial other than war and even then they would have off world colonies. Reduction in food output can take decades if you are still growing it naturally, more than enough time to come up with solutions. It's not like this was set anywhere near the heat death of the universe. Even then you have devices that can reverse entropy rendering that moot.
The BIG reason why we have to examine his "Plan" is because we don't buy his character or what little there is. His plan comes up nothing. If neither of those work what is left about him? Nothing but a punchman target.
There are some half bakes hand waves about how killing half somehow improves things but to avoid endorsing genocide they don't show anything. If there was some justification, something to show us his plan "Worked" or improved life one could see him as a monster instead of the biggest purple dumbass of the universe. It really doesn't help that the creators tried to play both sides by trying to have us "Feel for him" which rang completely hollow just like his character.
Killing people for Death would have worked perfectly fine if a little saucy given we have stories like Helena of Troy.
^^^
Walter White is the go to example of how to write a compelling protagonist villain that you can understand and relate even after he crosses your personal moral event horizon, truly becomes a villain, a villain with a believable path he took to get there. His journey isn't complete though as now we look to his comeuppance his twists and turns to avoid this. Yet he still does good here and there to stop his character from collapsing into a villain black hole which would take the show with it as you fully disconnect. There is some small hope that he will stop himself.
His intelligence and education gives him the means. His impending death gives him nothing to lose and motivation. He gains clear material benefits for himself and family. His plans are believable. His motivations are clear at every point. His choices aren't crazy, they are amoral, horrifying, logic taken to the extreme, done for attainable needs and wants.
Calling him crazy is outright wrong let alone reducing him to this.
By defending/accepting Thanos as a "Mad" titan only confirms how poorly written he is. Just because it's a punchman movie doesn't mean you lower your standards and excuse whatever is dumped in front of you. You should demand better, anything less is a disservice to yourself and entertainment as a whole.