Thanos is Marvel's most notorious villain seen in The Infinity Saga (or phases one, two, and three). He first appeared in The Avengers in 2012 and was last seen in Avengers: Endgame where he died not once, but twice at the hands of both Thor and Tony Stark. Thor used his new enchanted axe Stormbreaker to cut off Thanos' head after finding out he had destroyed the Infinity Stones, but Thanos is so bad that he needed to die twice. When The Avengers traveled back in time, Thanos hitched a ride (or rather was confronted in another timeline) and was brought back with them. He was later killed a final time by Tony Stark, who was wielding the Infinity Stones through the Infinity Gauntlet.
The Infinity Gauntlet was first used by Thanos to turn half of all the universe's population into dust with a single Snap of his fingers. This Snap would affect every sentient life in the universe. Thanos has always maintained the ruse that his purpose for this was to avoid the consequences of overpopulation and create a universe free of suffering. Thanos supposedly believed this massive genocide would bring stability and balance to the universe because overpopulation would inevitably lead the inhabitants of the universe to use up all of the available resources, which would condemn them to a life of starvation then extinction. And although it's a good lie, Thanos had a more nefarious reason in mind.
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One Reddit user has a very convincing argument that suggests Thanos did not care about ensuring enough resources, but rather he wanted to be a Leviathan (as described by the philosopher Thomas Hobbes). A Leviathan is someone or something that has power over everyone and everything due to the awe (or fear) they inspire in others. The Snap certainly would have created enough fear within everyone in the universe to turn to Thanos as their leader, and unite as one under Thanos' law, to preserve peace for as long as the memory of the genocide lasted. Consider this quote from Thomas Hobbes' book, "this makes it obvious that for as long as men live without a common power to keep them all in awe, they are in the condition known as ‘war’; and it is a war of every man against every man."
As the Reddit user points out, a war of every man against every man is an Infinity War that won't stop just because there are more resources; there needs to be an event that unites them. Thanos knows that The Snap will demonstrate his absolute power and show everyone that it is his will that is now law. Like an allegory to the Bible, this sudden mass genocide of half of all life would inspire enough fear for generations to come, as fear is easily spread and passed on, and a fearful society is more easily controlled. But this mass destruction (although it inspires fear, too) inspires resentment and hate—which means there can never be true peace, at least not this way—something Thanos did not know until after The Snap. This is why Thanos looks defeated in Avengers: Endgame after The Snap when Thor, Captain America, and Iron Man confronted him in the Ambush on Thanos.
Although Thanos had died, five years later the pain of losing half of all life was still just as fresh. So, The Snap seems to have worked a little (although not the way Thanos intended) as this fear and existential dread would probably have lasted at least several hundred years had then Avengers not gone back in time to retrieve the Infinity Stones. But even after this catastrophic event, without Thanos (or another leader) to look to in awe, the 'war' of every man against every man would have started up again, according to this theory. Thanos must be a really good liar if this theory is true, as he is constantly saying things like, "it's a simple calculus. This universe is finite, its resources, finite. If life is left unchecked, life will cease to exist. It needs correcting. I'm the only one who knows that. At least, I'm the only one with the will to act on it." Correcting here can be interpreted as punishing, another allegory to the Bible.
Basically, this theory about the true reason behind Thanos' Snap is saying that the universe will constantly grow and progress until it can't anymore, but it needs to feel defeated and brought down to its knees. That way, when it begins to grow again, there will be a sense of peace and happiness. With half the universe gone, those left are able to thrive without as much competition for survival. Those left alive will feel the safety one would feel knowing they don't have to fight so much for security of job, food, and shelter because there is enough resources to go around. And (according to the theory) Thanos believed that by giving the remaining population a sense of security and thriving life, that it would be enough to have them follow him out of fear and awe for what he "accomplished."
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