A recent second viewing of the first Guardians of the Galaxy film has brought Thanos's seemingly genuine words into question, as well as his claim that his culling of Gamora's home planet helped them prosper.
When the Guardians are arrested on Xandar, Gamora's scan reveals that she is the last surviving member of her people.
The Last. Surviving. Member. Which means that Thanos, even if he did only kill half the population, succeeded in wiping them all out.
Though this may just as likely be a continuity error on Marvel's part, Thanos's whole shtick doesn't add up when you get right down to it. He didn't wipe out half the populations on the planets he attacked; he wiped half out after killing thousands in the first invasion. So, half plus a couple thousand.
When he retrieved the gauntlet from Eitri, he didn't wipe out half the dwarves. He wiped them all out, except Eitri. Even if they weren't the entire dwarven race, they were still a separate planet all their own. So clearly his obsession with balance took a convenient backseat in this instance, with him effectively killing the dwarves' livelihood with the forge shut down, which would have definitely led to their destruction if he didn't already murder them all.
Moreover, when he snaps his fingers at the end of Infinity War, does it wipe out half of the half populations he already culled beforehand? Does that mean he's gone and made the population even smaller, again going against his own beliefs?
The hypocrisy is pretty blatant there, unless Thanos is really just that bad at math, but does Thanos, in fact, believe in his own propaganda, or is he just an excellent liar, so much so that he bamboozled the entire audience into thinking his cause was justified? He did, after all, state that he was a fantastic liar in the film, when he mentions he never taught Gamora how to lie, thereby making her quite bad at it.
This, of course, doesn't take away from how good a villain Thanos is. In fact, if he's just that good a liar, either because he's lied to everyone else or because he's lied to himself enough that he believes in his own cause, then that makes him an even better, more conniving, and more terrifying villain in the long run.