In the comics written by Jim Starlin, and all that came after him, Thanos's motivation is to kill and in doing so win the affection of Death, whom he wishes to take as a partner. It's the old princess-and-the-pauper storyline, save that the pauper is a mass-murdering genocidal madman and the princess is the cosmic and elemental force of death.
His motivation is one of selfishness and self-serving. He is the bad guy. In the Infinity Gauntlet series, his aim was to gather the Infinity Gems to wipe out one in every two life form in the cosmos as an offering to Death.
And in the Infinity Gauntlet series by Jim Starlin, George Perez, and Ron Lim, that's exactly what he does.
In the Avengers: Infinity War movie, Thanos has the same aim. He has been systematically culling populations on planets, but gathering the Infinity Stones would accomplish that by clicking his fingers — and wipe out one in every two living beings in the universe. However, his motivation has changed. While it has the same end game, the destruction of life at a colossal scale, one in every two beings, from his point of view, it is an altruistic act.
A culling of life so that it can survive and prosper. He states that it would be a fair and equitable act, rich and poor, religions alike. He cites his homeworld Titan as one that ignored his suggestion when he lived there and brought waste and destruction upon the world, as the population outgrew their environment.
As outlandish a concept as it sounds, it is one that has been espoused by respected figures such as the naturalist and environmentalist Sir David Attenborough, saying:
"They've been having… what are all these famines in Ethiopia, what are they about? They're about too many people for too little piece of land. That's what it's about. And we are blinding ourselves. We say, get the United Nations to send them bags of flour. That's barmy."
Bill Gates has talked about reducing population growth through increased health and seeing people choose to have fewer children as the threat of child mortality drops away.
"The number of people living on the planet has never been higher, their levels of consumption are unprecedented and vast changes are taking place in the environment. We can choose to rebalance the use of resources to a more egalitarian pattern of consumption … or we can choose to do nothing and to drift into a downward spiral of economic and environmental ills leading to a more unequal and inhospitable future."
Paul Ehrlich, professor of population studies at Stanford University says:
"How many you support depends on lifestyles. We came up with 1.5 to 2 billion because you can have big active cities and wilderness. If you want a battery chicken world where everyone has minimum space and food and everyone is kept just about alive you might be able to support in the long term about 4 or 5 billion people. But you already have 7 billion. So we have to humanely and as rapidly as possible move to population shrinkage."
And there are many conspiracies afoot that world governments are deliberately creating infertility or plain poisoning vast swathes of the population with vaccination programmes to achieve such a thing.
So it is in this context that we have Movie Thanos's belief, to do so without pain or discomfort, to simply wish away half the people (not enough for Paul Ehlrich) and then rest.
When he was just a bad guy, the Mad Titan mooning after Death, his madness explained his actions. Here it's having concluded that this is the only way. He is doing it, not out of a selfish love for death, but a love for life. And his opinion is shared by respectable, successful people and bodies that actually exist. And that's far more chilling.
But despite the differences between the Thanoses, they are going to sell an awful lot of Infinity Gauntlet collections after watching this film.