Thanos did nothing wrong - except for his population modelling.

In the Marvel fictional universe the character Thanos has the idea the kill half of the life in the universe - by his own statement because there are too many of them, because everybody left would be better off. The overpopulation and then collapse of his own civilization on his planet of titan led him to this philosophy.

He proposes to do this by random choice, because he considers this most fair.

This proposal has several problems:

1. A population at half its capacity is the most flexible - that is true for logistic growth with a fixed carrying capacity. The problem is that the population growth is also highest there. Which means that one soon has the same situation as before if one not also tackles the underlying problem of "How did one get to overpopulation?"
2. There are a lot of schemes fairer than random. My favorite is allowing every person to gift its 50% chance of survival to another person.
3. Why kill non-sentient life too? Those population will relax or die out anyway without anyone suffering for it.