While Joe Russo was speaking specifically about Thor during this portion of his Avengers: Infinity War conversation with Comicbook.com, this rationale also applies to Peter Quill, a.k.a.
The whole plan went to hell when Peter retaliated against Thanos, which broke Mantis’ hold over the Mad Titan and allowed him to return to full power.
When he, Drax, Mantis, Nebula, Spider-Man, Iron Man and Doctor Strange got the drop on Thanos on Titan and nearly succeeded in pulling the Infinity Gauntlet off the villain’s hand, when Peter learned from Nebula that Thanos had killed Gamora so he could get the Soul Stone, he became enraged.
Unfortunately, that decision led to trillions of lives being lost.