A/N. I know there are probably about a million interpretations of the beach scene out there, but I just couldn't resist trying one myself. And as I said, it's an interpretation; I don't own it. Lucasfilm and Disney do. So, with the disclaimer out of the way, here we go.
Cassian had been fifteen years old, lying awake in his bunk one night after a nightmare, when he had come to an uncomfortable, but inevitable realization. It was a realization that would stay with him for the rest of his life: the realization that he would become one of two people.
Number one: He would never see peace, for quite a simple reason: He would be dead. Although skill, a good amount of luck, (and though he hated to admit it, K2-SO), had managed to keep him alive all these years of rebellion against the Empire, a man could only run so far.
Or, number two: He would see peace restored to the galaxy, but he would never feel it himself. He couldn't. He couldn't after he had heard the screams and pleas of so many dying men: After he had heard enemies gasping for one last breath to either plead for mercy or curse him as they lay on the ground in his blaster sights. After he had seen comrades' chests wheeze up and down for a final time before they went still in his arms. After he had felt limp bodies, once people who had trusted him, sag into his arms with burns from his blaster searing their backs. Cassian had needed to decide who to kill, who to leave behind, who was most important to their mission, and who was expendable. When he was haunted by nightmares and that intimate knowledge of lives lost (lost by his choice), how could he know what peace was? How could he enjoy it?
Now, collapsed on the beach of Scarif with his leg throbbing underneath him, Cassian knew he would be the first. He was going to die. He would never see the peace that he had been fighting for since he had been six years old.
Only, he did. His fingers had made deep indents in the sand as his leg burned with pain, and he slowly raised his gaze from them to stare into Jyn Erso's eyes. There, he saw the knowledge that they had not failed and the satisfaction that whatever mistakes they had both made in their lives, they had done this one, most important task right. They, a rag-tag little team of rebels, had stolen the Death Star plans from under the nose of the powerful empire. Jyn had made Galen Erso proud and saved Saw's dream; She had done what she could, and now the task had been passed into other hands to be carried on. In Jyn's eyes, he saw peace.
He wasn't the first choice, Cassian realized. And his fifteen-year-old self had been wrong, because he now knew he wasn't the second option, either. As he wrapped his arms around Jyn and held her close, waiting for the light to wash over them, he felt peace.
