The Guardians later find Thanos on Knowhere as he intimidates The Collector for the Reality Stone, and then leaves with not only the cosmic artifact but also with Gamora. He realizes his adoptive daughter knows the location of the Soul Stone, but kept it a secret from everyone but her sister Nebula (Karen Gillan). It's only when Thanos begins to torture Nebula that Gamora takes him to the planet Vormir, where she's killed in exchange for the Soul Stone, which requires the sacrifice of someone you love -- a soul for a soul -- before it can manifest.
Back on Titan, with Thanos restrained due to the efforts Doctor Strange and Drax, Mantis uses her empathic abilities to lower the Mad Titan's temperament, providing an opportunity for Iron Man and Spider-Man to rip the Infinity Gauntlet from his hand. Of course, Star-Lord immediately questions Thanos about Gamora's whereabouts, and it's not long before Nebula reveals that her sister was killed by their father. Enraged, Star-Lord attacks Thanos, inadvertently freeing him in the process.
With their last shot at victory gone, Thanos takes out the heroes and delivers a near-fatal blow to Tony Stark. Star-Lord's mistake forces Doctor Strange to make a major concession in order to save Tony's life: He hands over the Time Stone. From there, Thanos teleports to Wakanda, takes Vision's Mind Stone, snaps his fingers and wipes out half of life in the universe -- all thanks to Peter Quill's rage-fueled misstep.
It's understandable for Star-Lord to want revenge for Gamora's death, but his temper tantrum came at an unimaginably high cost, not only for the MCU's heroes but for the entire universe. If Peter really wanted to end the threat of Thanos once and for all, he could have simply waited until the Mad Titan no longer possessed the Infinity Gauntlet, and then killed him. No one is suggesting Star-Lord couldn't have his vengeance, but timing is everything; just a few more seconds, and the good guys win. But instead, Peter is wiped from existence along with half of Infinity War's stars.
Fans will have to wait another year for Avengers 4 to see whether Star-Lord is able to redeem himself after his colossal mistake. But for now, just know that Thanos wouldn't have won if not for the help of his greatest ally, Star-Lord.
Directed by Joe and Anthony Russo, Avengers: Infinity War stars Robert Downey Jr., Chris Evans, Chris Hemsworth, Mark Ruffalo, Jeremy Renner, Scarlett Johansson, Paul Bettany, Anthony Mackie, Paul Rudd, Elizabeth Olsen, Tom Holland, Benedict Cumberbatch, Chadwick Boseman, Chris Pratt, Zoe Saldana, Dave Bautista, Bradley Cooper, Vin Diesel, Tom Hiddleston and Josh Brolin. In theaters now.