Then honestly, I gotta say you simply weren't paying enough attention.
It was all but announced during the Hulk fight: he wanted Hulk to feel victory before he took it away from him. It was something he enjoyed, and he kept doing it. It probably stems from what happened on Titan. Probably a capable leader who felt he had a great solution, only to have it taken away from him at the last second. It's his means of revenge and what he was doing all movie long.
I mean even the scene with Gamora and the soul stone, he let Gamora believe his quest was over because he loved nothing, before telling her "I love you" by trading her life for the soul stone. Every single time he would secure a victory in the movie, he would first let his opponent think they had. Then he would take it away.
It was kind of the main theme of the character so I'm at a loss when understanding how anyone missed that. That was Thanos in a nutshell: making you believe you'd won before taking it all away.