(more infinity war spoilers)
Oh absolutely Peter Quill makes a mistake in this movie by allowing rage to overtake him at the last possible moment, but he doesn’t really make any more mistakes than any other MCU hero made in this movie/the leadup to it. And what happened might well have happened anyway without him making said mistake? So no, he’s not the other villain of the movie, AV Club! Jeez! And also a slight ‘ugh’.
I’ve become very protective of Peter since GOTG2. He’s an abused kid who grew up with nothing, lost so much, and has to live out the rest of his life knowing he was essentially the product of a megalomaniac god’s creepy breeding program. Now he realises that Thanos has sacrificed Gamora in almost the exact same way his father sacrificed his mother? Of course he’s going to react like that! It wasn’t done out of malice, just pure in-the-moment rage for everything that’d been taken from him.
(And yeah, I doubt that the plan would have worked even if Peter hadn’t snapped. Mantis is strong but Thanos is nigh-on indestructible.)