Thanos is arguably a lot more interesting in Infinity War than he is in Endgame. Played by Josh Brolin, Thanos is the “hero” of Infinity War, and you see things from his warped viewpoint of reality. But it actually makes sense when you see it from his eyes. Wouldn’t the universe be better if half the people in it were gone? It would mean people wouldn’t starve, and that there would be more space for the survivors.
Of course this is a load of crap since it would mean that countless people would just cease to exist, but it actually seems reasonable in Infinity War. It only seems like the genocide it is when we get the heroes’ perspective in Endgame. But the fact that we all went along with what Thanos was saying is evidence that he was super convincing, and for that reason (and the fact that two whole phases were built up to him) Thanos is the number one villain in not only phase 3, but the entire MCU.