They might give it more explanation in the second part but I totally bought into the idea, his planet was ravaged by overconsumption, he had a daring plan that would have saved it and was chastised for it. What then happened was exactly as he predicted. Instead of only losing half, greed caused them to lose everything. Obvioulsy he's a psycopath to add to that but he has made it his lifes mission to set the universe right and give it more time to develop.
Any plan a psycopath comes up with should involve a certain amount of cretinism. I think you're being a bit harsh on that supposed plot hole there. I mean Hitler supported the extermination or expulsion of people to create a lebensraum. Crazy people do crazy things.
But Hitler said he wanted rid of the Jews and set about killing them all. Thanos has set about a plan which would fail utterly to solve the perceived problem he's trying to solve. This is like the SNP's minimum alcohol unit price idea, which makes Thanos basically a big-chinned Jimmy Krankie