All right, let's imagine a version of Thanos who wants the bleeding obvious thing to make sure everyone's needs are met: producing food, shelter, medicine, healthy environments, etc. Now his utilitarian logic is: "There are people suffering all over the universe. I have to get all the Infinity Stones to stop the suffering. But all these jerks are trying to keep the Stones away from me. As much as I'd rather not have to hurt anyone to complete this righteous task, if I have to raze twenty interstellar civilizations to stop universal suffering forever, it's still a no-brainer. And I'm the only one with the vision and strength to make it happen."
Of course, there are some who think that sounds great and follow Thanos whole-heartedly. Others just want to side with a winner rather than be ground under his boot. But the most dangerous threat is that Thanos creates doubts and uncertainty in the masses of ordinary people who the heroes need to step up: "Are we throwing away billions of lives and vast resources in this ghastly war trying to stop this guy from feeding and sheltering and curing the whole universe? We're literally dying in defense of suffering."
And just when the audience is most ready to concede to Thanos's vulgar utilitarian logic, the heroes appeal to something higher, and they inspire the people of Earth and other worlds to show courage in the face of his intimidation.
The heroes want to alleviate suffering too, but they know it has to be done with accountability and checks and balances, respecting people's incalculably complex desires. (We're not just mouths to feed and bodies to keep warm and safe.) The civilizations we've built, for all their flaws, have done a lot of good work in that direction already, and it would be a fatal mistake to risk all that on the promises of a saint, much less a violent monomaniac. (Here the movie could reflect on how Iron Man learned that lesson from his attempt to save the world with Ultron, and tempered that lesson in Civil War. If you're going to end the conflict with his sacrifice, make it the culmination of his growth as a character.)
See, even if we're not 100% sure of our course, we simply can't trust this guy's promises to just end suffering and not abuse all that power. No matter how seemingly well-intentioned and plausible his arguments, one person with so much unchecked power is a catastrophe waiting to happen, and it's especially foolish to trust someone who doesn't balk at the idea of trampling over billions of people (their freedom, their lives, and all they've built) to achieve his goals. What happens when he gives people stuff and it doesn't turn out exactly like he hoped? If people are still unhappy or ungrateful, what would a person eager to use unlimited power do to all those 'broken' people?
We can't hand off the responsibility to a strongman, nor stand by and let him seize it from us. We have to step up.
With a small change, the villain's logic becomes more compelling, and the conflict itself becomes more compelling.
