It's an unfortunately fashionable thing these days to try and claim someone who isn't the obvious villain of a movie is actually the most evil one in it. Spend enough time hanging around the murkier levels of the Internet's film theory communities and you'll see that pretty much every major film hero has been accused of being the actual villain of the piece. Conversely, a significant number of villains are said to actually be not as bad as they seem by the same sort of theorists. It's hard to keep up.
The latest hero currently being pilliored for his part in Avengers: Infinity War is Star-Lord, who has been widely criticised by fans and even some journalists as the real villain of the piece. You know, despite the fact that an actual space Hitler intent on wiping out half of existence is clearly the baddest guy in there.
It's even got to the point that Chris Pratt has been hounded on social media (thankfully, he's taken it in good spirits), as if that's every ok. And the reality is, Peter Quill deserves precisely none of the accusations. He's not a villain, he's human.
The suggestion that he's a villain stems from the fact that it's he who messes up the plan to remove the Infinity Gauntlet from Thanos' arm when he demands to know where Gamora is and finds out Thanos has killed her. Naturally upset, he takes out his pain on Thanos' face and the Titan escapes the collected hold of the Avengers and Guardians.
Cue the exasperated reactions of lots of fans and snarky journalists. But you'll have noticed that none of his fellow heroes blamed him. Even Tony Stark, who openly blamed Doctor Strange for giving Thanos the Time Stone too easily said nothing.
Presumably, in Doctor Strange's millions of potential outcomes, there was probably one where Star-Lord didn't distract Mantis enough to break her control over Thanos. It's simple probability. And the fact that Doctor Strange did nothing to stop Quill attacking Thanos - when the reality is that his powers would allow him to do any number of things - suggests that Strange knew this needed to happen. It was part of the single future in which the heroes win.
But even putting that aside, Quill's reaction to Thanos is entirely understandable. It doesn't make him a villain at all. Think about it: Quill's history is one of immense trauma. He was torn from his family as a child and basically forced into the servitude of pirates, his mother was killed by a cancerous tumour put there by his absentee alien father, who then turned out to be a villain and tried to kill him and the only real father he ever knew - Yondu - ended up dying as well. He might joke about it with Thor, but his life really has been a sh*tshow.
He's been isolated for most of his life, building up emotional armour (in his sarcastic attitude, his quickness to humour and his promiscuous romantic entanglements) and all of a sudden he's faced with a legitimate relationship with Gamora. It isn't the same sort of thing he's been used to (his Captain Kirk-like dalliances around the galaxy), this is the real deal. He wants to be her dance partner.
And just as the relationship is blossoming after some flirtations, she's taken from him just as everyone else is. But there's no caveat to how he feels about her. He may have been taken from Earth, but he enjoys what he became. He may have lost his real father, but he was a murderous Celestial. He may have lost Yondu, but he was a pirate lord who definitely mistreated him (just a little more affectionately than he might have others). He may have lost his mother, but he was young and the memories have been pushed away. With Gamora, there was no but.
So, of course he would react like that. That was the most human way to react possible and as his rejection of Ego's evil plan confirmed, Quill's humanity is what defines him rather than his strange alien biology. Not reacting to the news of Gamora's death would have been a betrayal of him as a character and their dynamic as a pair: the Russos didn't turn him into a villain, they simply turned up everything that makes Star-Lord Star-Lord.