This is actually just a random one-shot death scene that I wrote when I was really sad. So I apologize for it. Basically Lucifer was a serial killer taunting Dean and Dean and Cas had a huge fight about it the night before. But Dean's finally got the chance to take Lucifer in for good.
"Dean I told you I would take your life from you. I promised you didn't I" Lucifer's mouth curled in to a smile. He fired his gun. Dean ran at Lucifer and shoved him in to the wall, he turned him around and pushed his chest in to the wall, grabbing his arms and handcuffing him. Dean pushed in close to him and breathed in to his ear "Too bad you missed asshole" Lucifer let out a chilling laugh and it made Dean's blood run cold "I wasn't aiming for you Dean"
Dean's heart stopped in his chest. He wouldn't have followed him in would he, he told him to stay outside and he wouldn't do it would he. Dean looked over his shoulder and what he saw made his heart stop. Dean pushed his knee in to back of Lucifer's legs and forced him to the ground and he thrust Lucifer's head in to the wall to knock him unconscious. Dean ran over to the body lying on the ground. He pulled off his jacket and pushed it in to the wound on Castiel's stomach.
"Dammit Cas! Why couldn't you just listen to me this once and stay in the damn car" Cas let out a small chuckle, he put his hand on Dean's chest. Dean grabbed his hand and smiled down at him "You're gonna be okay, you're not gonna die on me because you can't" Dean grabbed his phone and called Sam "Sammy I need you to send in the paramedics we've got an officer down"
"Dean, listen to me, this wasn't your fault. I know you, and I know you're gonna blame yourself but this was my choice…" Cas coughed and had to spit out the blood that had pooled in his mouth "I need you to understand that Dean, but I also need you to know that if I could stay…" Cas tried to continue but it was getting harder to breathe. Dean grabbed his hand and pulled out the box he purchased earlier.
"Cas I was gonna do this last night and I'm sorry I didn't but I need you to know now that I am never going to love someone as much as I love you…" Dean had to swallow as the tears fell down his face, he knew Cas wasn't going to make it, the blood was pooling around his stomach and soaking his shirt, Deans hand was stained with his blood "Castiel you make me want to be a better man, and you are my life, would you become my husband?" Dean took the ring out of the box; it was a black band with angel wings carved in to it. Castiel stared at the ring it was beautiful and he couldn't imagine a ring more beautiful "Yes" Dean leaned down to kiss Castiel and put the ring on his finger. Castiel smiled up at Dean and tried to talk but Dean shushed him with another kiss. Dean could hear the paramedics in the distance. Castiel was still holding his hand but he could feel the grip weakening, and his eyes were only half open. "Do you know why I chose angel wings?" Castiel let out a small no. Dean smiled he'd never really told Castiel that he thought of him as his angel but he had meant to "Well…erm…in high school when we first met, I never told you this but I was supposed to fight that asshole Zach and you turned up at that moment about fucking biology and I swear I was never more thankful that you were my lab partner. Then after being partners we never really spoke till we met in that bar in Junior year, you were the last person I thought I'd see there but I did and once again you stopped those guys from beating the shit out of me…and I remember thinking this guy has to be my guardian angel or something" Dean chuckled at his own thoughts and he smiled at Castiel, Castiel smiled up at him but it was small and weak. Castiel's hand dropped from Deans and his breathing was slow and shallow. The paramedics opened the door and were rushing towards Dean and Castiel. Dean looked down at Cas and wiped the tears that had slid down his face. Dean leant down and pressed a chaste kiss to Castiel's lips and he knew that it would be the last one they would share. He moved back and looked down at Castiel, his eyes were closed and his chest was no longer moving. Dean moved his hand to his neck and couldn't feel the pulse he hoped for. His tears came freely, falling down his cheeks and on to Castiel.