I liked the movie. It did a really great job of bringing together all the accumulated characters and story arcs in a meaningful and satisfying way. Great fight scenes. Implausibility only becoming physically painful in the scene with stellar re-ignition via elbow grease.
The movie's most notable success was subverting expectations by making the genocidal big-bad into an empathetic and rational protagonist. In fact, the central protagonist of the movie.
The problem:
Thanos' plan, through the whole Avengers timeline, is to wield the combined potency of all the 'infinity stones' to wipe out half the population of the universe in an instant.
His motivation for this is to save all civilisations from implosion via resource depletion, brought on by over-population. As happened to his home world, after he personally failed to convince them to randomly cull themselves.
It doesn't really matter that this is a stupid plot mechanism, in that...
(a) Caveman or medieval civilisations, elsewhere, aren't in any danger of exhausting planetary resources. Maybe we should assume he only culled human level, globalised civilisations. (But then also, what about the various, seemingly stable societies we see in Guardians and Thor?)
(b) Clearly a one-time cull is only going to delay the inevitable by a couple of generations (if it is inevitable).
... It's made to work in the movie. There is suspension of disbelief.
The dangerous part is that the only reason this plan is presented as undesirable is the killing itself. "Murder = evil" is such a universally evocative idea that no one can avoid making that the main counter-argument.
But that's not enough in reality, where the ends tend to justify the means: we have large, expensive organisations dedicated to making and utilising weapons to kill potential threats to our civilians and nations.
When we, as humans, feel our lives are threatened, we tend to acquiesce to amoral measures. In the last decade, nationalist xenophobia has been fuelled by increasing financial and physical hardships on individuals. (Not mattering that this is primarily from unrelated, rising wealth inequality.) This collective sentiment has already enabled callous right-wing/authoritarian politicians to implement horrible policies that would normally have been seen as too inhumane.
With further, deeper economic crashes seeming ever more pressing (in the West), the tale of woe so far could easily be eclipsed, in terms of increased suffering catalysing far further reaching humanitarian disasters, even war, etc.
Add to this ferment the popular notion of global over-population, with such bastions of scientific authority as Sir David Attenborough pushing discussion of the need for population reduction.
When bellies are empty and people are terrified, maybe a little genocide can slide, if it's helpful in the grand scheme of things...
Why it's wrong headed - because it won't work!:
Morality, even in the extreme, is malleable. Practicalities, not so much.
