In the movie, he mentions that Gamora's homeworld was on the brink of extinction due to overpopulation, and that since his intervention and subsequent half-genocide, it's a paradise where people have all they need.
If you consider Thanos is speaking the truth at this time, it means that after his genocide, Gamora's people figured they needed to take measures to prevent their population from growing too much or else Thanos would return.
Here he does it to every world simultaneously in the universe without going himself, but his reasoning is that once every world has felt the impact of losing 50% of the population, they're going to take measures to prevent it from happening again (better ressource management, control of rampant birth rates, etc...). Of course that's only if the populations are aware of WHY it happened, but I assume Thanos could just conjure a microphone out of thin air at this point and speak to the universe to let them know his way of thinking.
Doesn't make him not a murderous wacko though, he's definitely not a "good guy", not even a anti-hero. He's a raving lunatic that BELIEVES with all his faith that he's doing what needs to be done, and it's this absolute conviction that makes him sympathetic because he feels like he has to do it, and hates that he has to sacrifice everything for it but it just must be done.
