User blog comment:Looperreallyreallysucks/Pure Evil Discussion - Avengers: Endgame/@comment-29414935-20190513140654/@comment-24097388-20190513174547

Because he's come to the conclusion that being nice and reasonable will not work, even if he spared the lives of others. At the end of the day, they won't accept what was destined to happen. You can make the argument that he's doing it out of spite, but what is fact is that he's doing this not out of pure greed and selfishness.

And even though it happened in another timeline, it still showed what Thanos true feelings were towards Gamora has been all these years; a man that loved his daughter. Those tears were proven to not be fake in the slightest.