Cassian has always expected to die in the field. The life of a spy doesn’t lend itself to peaceful retirement and happy endings. He’d always assumed that his end would come in battle, burned away by the fire of blaster, or in an Imperial cell, carried gently on the wings of his lullaby.
Death meant failure — of the mission and maybe of the Rebellion.
He’d never expected his death to be peaceful, but kneeling here on the beach, the thrum of a successful mission in his veins and the comfort of another person to hold him at his end, he thinks for the first time that he can face death without fear.