Quill didn't quite know what to feel at first.
Then he realised what the feeling was, the same feeling he had when he learned the truth about his mother, anger, denial, not wanting to even believe the unthinkable has happened.
Don't make the mistake again Quill.
He let his anger get to him when Ego told him the truth.
The tumour.
His mother...
Thanos is a God, he'll kill you like he did to Gamora, what use would your guns be?
If he died fighting Thanos, then he'd die with Gamora being the last thing in his mind. Her beautiful eyes, her furrowed brow when he talked about Earth, wanting to learn all about his childhood and heroes; he never explained to her that Kevin Bacon was an actor, but dammit he was a hero to him.
Don't be a god damn hero, Quill. It's not worth it. They're yelling at you, telling you not to do it. Tony, that magician guy, Drax, Mantis, that kid with the spider costume, they'll be in danger because of you!
So what...
If Quill died, he'd see Gamora again, the one person who he didn't have to act as "StarLord" to. She saw Peter Quill, a man who thought he lost everything, who only thought he'd have to count on nobody but himself.
How wrong he was...
Gamora was the one he needed...
Would killing Thanos bring her back?
Of course it wouldn't... he'd die. This would kill him...
But he'd see the ones he lost once again, his mother... Yondu... Gamora...
If seeing them again meant that he had to shoot this bastard in the face, then so be it.
Memories of Gamora hit him, listening to his music together on Knowhere, the looks they gave each other on the ship, that unspoken bond between them; he knew there was something and he was right.
That kiss! They finally kissed and Quill thought like he could die and that moment and he'd still be happy.
He'd be happier if he did die just then...
He'd be happier dying before Gamora.
He'd be happier not being here without her to keep him in check.
Quill
He reaches for his gun.
Quill!
He ignores the shouting, the begging, they have Thanos right where they want him, but they aren't gonna be the ones to kill this bastard.
Don't do it Quill!
He shoots.