Are we missing the fact that he's the MAD TITAN here? You're basically second-guessing someone who demonstrably has a really effed up moral compass.
No. I'm noting that they don't actually establish his madness in the movie! He's presented as
entirely rational.
Every leader in history who has presided over war has had to do the calculus of, "Is the end I/we seek worth the lives it costs." The vast majority of them are not considered mad - so being willng to have people die for an end is not, itself, sign of madness. If he is correct, that the future of the Universe is doomed unless the excess population is culled, then he may not be considered "mad" at all, as he is saving people in the long run, and is at worst Maciavellian in his appraoch.
He even presents a pretty solid example - his homeworld, that was consumed by its own population. Again, his conclusions that something drastic must be done to save all worlds with sentient life seems supported by evidence!
He only becomes mad when we make it abundantly clear that his position is not rational, that he is moving contrary to reality. And they don't actually do that in the movie!
Of course, there may be all sorts of other limitations on what the stones can actually do. Can they really create resources on the same scale as they can annihilate life forms? Can they actually enlighten every sentient in the universe?
They just spent the past decade establishing how immensiey powerful these stones are, and now we are supposed to just accept that they have criticaly plot-relevant but unstated restrictions? If the Stones have such limitations, that needs to be established
in the movie to be a strong argument.
The stones are clearly capable of things other than destruction - see the Vision, who was effectively given life/sentience by one. Thanos uses them to create very realistic ilusions. From the Snapocalypse we know he can reach at least half the population of the Universe simultaneously. The suggestion is that the fact that he limits to half is his invention - he was killing half of worlds before he had the stones. So, he can reach everyone. He has Mind, and can reach everyone. How should he *not* be able to enlighten everyone?
And again, this would all be fine if
anyone rejected his thesis, or suggested alternatives, such that we could see his devotion to this one course is irrational.