There was a method to Thanos’ madness in Avengers: Infinity War.
Thanos in Marvel’s Avengers: Infinity War was a supervillain that was never before seen in cinema. He was a well-rounded character and had believable — if insane — motivations and a purpose which he wanted to achieve no matter what. And in the end, he did exactly that when he extinguished half of the universe’s life by just a snap of his gauntleted fingers.
But still, one thing was missing. The Mad Titan, as he is called in the lore, did not have any particular malice against the Avengers and their allies. He was utterly deranged, but still somehow thoughtful. There was a method in his madness. Personally, too, he was not exactly a typical movie monster who is driven by the hatred against the hero. When he did kill (Loki, Gamora, Vision and so on), it was because there was a purpose.
Beyond them, he did not kill any superhero, which is strange. He did not even kill Thor, who nearly killed him, and just escaped to an undisclosed planet after his goal was achieved. Why?
Marvel Studios’ Head of Visual Development Ryan Meinerding explains in The Art of Avengers: Infinity War. He says, “The plan of getting all the Infinity Stones is putting [Thanos] in the position of being so powerful that he is sort of calmer, more single-minded, and more reasonable. He is not necessarily worried about killing the heroes. As long as he ends up with the stones, he can accomplish what he wants to.”
The heroes are just annoyances for him, a bunch of flies he can just swat away. He does exactly that and does what he set out to do. Although about half of Marvel superheroes do die, it was, as Thanos said, random and dispassionate. It was not pre-decided that they will die. They just happened to be the ones who did.
Avengers 4, releasing next year, is expected to undo the damage. The superheroes who died like Black Panther, Doctor Strange, Spider-Man and others are profitable properties that Marvel would not just abandon.