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. Star-Lord. 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. 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. And going back to Thor, having watched Thanos kill Loki, Heimdall and half of the Asgardian population at the start of the story, it makes sense to a degree that he’d want Thanos to know it was he who killed him. Unfortunately, that decision led to trillions of lives being lost.