Fooled around and fell in love.
He'd made mistakes before, they all had. Rocket and Groot had been bounty hunters for years, and had been all over the Galaxy. If that wasn't illegal, he didn't know what was. Drax had mercilessly killed hundreds of people, he was Drax the destroyer. Mantis, that was different. She had unknowingly helped ego for years. She was just so kind and sweet, and was manipulated so easily, that she didn't know. Gamora had made many, too. She was a deadly assassin, daughter of Thanos along with Nebula, her sister. She had also mercilessly killed hundreds, that's what she had tried to do to him when they first met. Granted, he was carrying a seriously deadly object, almost the most deadly thing in the universe.
But he had made the worst mistake of them all. He had killed them all. He had gotten everybody killed for his selfish needs, and certainly payed the price for that one. He'd been making mistakes for years. Working with Yondu as a Raveger, sleeping with every woman he met, stealing stuff for the fun of it. His mother would not be proud.
Sometimes he sat alone on the ship and just thought of her. Meredith Quill, his mother. She had died when he was just a child, coincidentally being the same night he was abducted. That night had devastated him. His mother was his world, and he was her little Star-Lord. He had regretted not holding her hand every second he was away.
"Why did I not take it?" He had said to himself one night, not being able to sleep. He had crept up to the front of the ship and sat in the pilots seat, just staring at the stars.
"Why couldn't I just take her damn hand and make her happy before she died." Tears made their way down his face as he poured his heart out to the empty air. He put his head in his hands and sat like there for a couple of seconds, before the sobs started to rip their way out of his chest, filling the silence with a deep sadness. Little did he know, somebody stood behind his chair, watching him with a frown on her emerald lips.
She slowly walked next to his chair and placed her and on his shoulder comfortingly, making him look up at her. He didn't even bother with an excuse, just looking back to the floor.
"Peter, you couldn't have known what was going to happen." She whispered, crouching in front of him. He shook his head.
"I knew she was dying!" He yelled. "I knew she was dying and didn't take her hand!" He looked to the floor again and closed his eyes, tears dripping from his cheeks and onto the floor.
"You were a child." She said gently, taking his hand and making him look up at her. "You were a child and you were scared. You didn't understand, and it's okay. She would be so proud of you if she were here." Gamora game him a small sad smile, earning one in return.
That was the night their relationship grew. Sure, he had always liked her, and he often flirted with her, but that's when she fell for him too. Groot was just a sapling at the time, and he hadn't remembered being the old Groot, so he guessed they were always together. He had acted like a father figure to Groot, and Gamora like his mother. Rocket was like a big brother, Mantis a sister, and...to be perfectly honest, he didn't know what Drax was to him.
Another night he needed her the most, was a couple of nights after the Ego event. He had lost his dad. Not the bastard of a father he had been almost killed by, but his true dad. Yondu had always been there for him. He made sure the crew didn't eat him, trained him to fly, to fight, to live, and he always thought of him as a bastard. He didn't see that he cared, he just saw the shit-face that abducted him.
Not even his funeral made him cry like the night after. He was sitting alone in his room, his mind too wild and active for him to sleep. Whenever he closed his eyes, he could see Yondu as he saved his life, giving up his own. Tears fell from his eyes as he really thought the events over. A sob tore from his throat before he could stop it, then the others soon followed suit. He sat there crying for a while, not knowing that Gamora was on the other side of the wall, his heartbroken sobs pulling at her heart strings. She had got the courage to go in there a while later and sat holding him, gently rocking him as he cried. The next morning, he had thanked her with all his heart for her being there, and she just smiled.
But, in whatever he did, she was there. When he needed a shoulder to cry on, she was there. When he got pissed and flipped out in front of everybody, she was there, whenever he needed her, she was there. Another mistake of his, not being there for her. He had been there for everything she told him about. When she decided to explain more about her father, he was there. It was late at night, and she had just gotten Groot to sleep. It started with the simple words of "He's so lucky to have two parents." And, of course, he had seen the look on her face and worried. She was always good at hiding her emotions. Unless she was angry, she kept them locked away. But that day had taken a toll on her.
They had gone to Knowhere to talk to some people, and as she waited in the bar for the representative, she heard whispers behind her.
"Daughter of Thanos, that one."
"Heard she's betraying her team for him."
"Yeah, said that she'll slit that bloke's throat."
He had tuned around to look at them, just as she had tore out of the bar, and went back to the ship. She had busied herself for the rest of the day, not wanting to talk about, but that night she had broken down. She had kept her emotions in for too long, and they had exploded to existence as he asked "Are you okay?"
She had told him everything that night, sat on the deck with blankets wrapped around each of their shoulders. As one point, she stopped speaking and just cried. Cried for the loss of her parents, the transformation of her sister, her abduction, and the loss of herself. Before Thanos, she was just an innocent little girl. But he had changed her into a weapon. The crew had found them in the morning, wrapped into an embrace so tight, so that he was practically holding her together.
She had been quite open to him then, and the same with him. But he wasn't there when she needed him the most. He wasn't there when Thanos forced her to take him to the corners of the universe. He wasn't there when he had grabbed her wrist painfully tight, and dragged her along against her will. He wasn't there when she fell, her hair blowing in her face as she screamed her throat raw. He wasn't there when she died.
But he was there when Thanos came. Four out of six infinity stones in his gauntlet, he stormed in and fought them, and they almost won. Mantis crouched on his shoulders, trying her hardest to keep him subdued.
"He mourns!" She cried out, looking panicked. That was the starting point for the madness. Not even five minutes later, Peter had screamed at him, tears rolling down his cheeks for the loss of his loved one. They almost had it. Peter had pulled the gauntlet off of his wrist, just as he started to punch at him, yelling everything that came to his mind, wishing that bastard was dead. He had killed his love, his soul mate, his Star-queen. But he grabbed the gauntlet back. He had fought again, and he had won. Peter stood there as he watched. Drax, Mantis, Peter, then him. He looked down at himself as his skin flaked into dust, then darkness.
So, yes, Peter fooled around and fell in love. His worst mistake, the one that lasted for infinity.