By the way, I do have friends that are very seriously arguing that Thanos was the environmentalist hero in this movie, and that he did nothing wrong.
Now, human beings perform Thanos' actions every hunting season - we know that a deer population would starve with overpopulation, so we allow hunting season to stable off the population. If you talk to some big name hunters, they will always say that they are in it for conservation. Why are we able to rationalize this with deer and not our own population? The only reason is that we consider ourselves to be the custodians of this planet, and a higher form of life than deer. Thanos considers himself the custodian of the universe and a higher being to any other life form. That makes his ethics morally equivalent to our own. He did nothing wrong by our own definitions of morality.