The darkness of this movie isn’t in the villain-protagonist’s victory. It isn’t in the deaths of major characters. It isn’t even in the obvious cynicism of those deaths as a set-up for the next film and their take-backs. It’s in the movie’s bleak view of love.
Thanos seizes the soul stone because he loves Gamora enough to make killing her a sacrifice. The heroes on Titan fail to defeat Thanos because Peter Quill loves Gamora so much that he lashes out over her murder rather than helping his teammates. Thanos can step back in time to pull the final stone from the Vision’s forehead because Wanda Maximoff loves him too much to risk his life by destroying it when she had the chance.
Love ruins everything in this movie and that fact runs contrary to a core tenant of the ideology of the action-adventure genre Marvel’s films sit nestled within: that in moments of danger, your love for a spouse, a child, or a buddy will give you strength enough to keep going, to do the impossible, to win.
Not this time.