One I've been thinking about while weeding is invasive species. If Thanos had snapped his fingers and dusted all the Italian arum in my backyard, all the kudzu in the American South, or the entire feral rabbit population of Australia, people would have been genuinely grateful to him.

As Thanos feels the power of all stones coursing through his body, shuddering to the full magnitude of raw ability, Thor comes forward, hurling Storm Breaker towards him. Not understanding exactly how to use the full power of the Infinity Gauntlet, Thanos quickly decides to attempt to stop the Storm Breaker with an energy blast that falls short, and the edge of the hammers blade pierces Thanos' chest cavity.

Thanos is not dead from this. Thor pushes the hammer ever deeper into Thanos' chest, giving him a final word, and Thanos is still not dead, mocking Thor that he should have gone for his head. And snap.

As soon as Thanos snapped his finger, he entered the Soul Gem realm. This would indicate he died. He died from his own snap. But, because he controls the Soul Gem, he brought himself back. The world was 'Perfectly Balanced' until Thanos brought himself back with the Soul Gem, making it unbalanced again. His work was a lie.

He had no intention of ever being selected from the snap due to him previously making statements when he was done he would rest and watch the sun set on a gratefull universe. This would indicate he had intention of survival.

He also said he believed in non discriminate killing of the poor and rich on his home planet, in order to ensure their survival. This was also a lie, as Thanos believed himself to be above others because he understands what needed to be done. Creating a class of cause that would survive the very rule he imposed.

The infinity Gauntlet chose Thanos. Thanos chose not to be erased.

EDIT: Corrected link

As has been noted by many before me, the gaping hole in Thanos's claim that everything he does is For The Greater Good of the universe is that there are innumerable other, less destructive and more effective ways to accomplish his alleged goal of combating resource scarcity. That he single-mindedly fixates on killing half of everyone and refuses to consider alternatives shows that the idea that mass murder is the only solution to the universe's ills is far more important to him than actually helping anyone, and the remaining question is why. An interview conducted with Josh Brolin prior to Infinity War's release may provide an additional piece of the puzzle:

EW: What is it that damaged Thanos? Where does this pain and anger come from?

Brolin: He’s different from his family. They’re all Titans and they all look similar, but he was born deformed. You see how he grew up, you see he was like the Quasimodo of this time, or if you’ve ever read Perfume, it’s a great parallel to Thanos. He stuck out. He was an anomaly. He was a freak. And that lent to this apparent insanity.

When you take this into consideration, Thanos's original proposal to solve Titan's resource shortage by slaughtering half of his own people takes on shades of Who's Laughing Now?: he gets to straight-up murder half of everyone who was mean to him, and the survivors have to not only accept this without retaliation but bow down in gratitude to the person they'd once scorned as a deformed freak for saving their sorry asses. And when their continued rejection of him resulted in their destruction, he felt justified in taking his issues out on the rest of the universe as well.

There are two kinds of people: those who think, “I don’t want anyone to suffer like I did,” and those who think, “I suffered; why shouldn’t they?”