They keep saying, "It's not your world", as though he could have chosen a home somewhere else in the universe, as if he left his home planet hell-bent on world domination, as though he himself doomed Krypton to its destruction.
They spit it with such venom, as though he provoked Zod and led him across the stars to Earth. As though he simply decided to come to Earth so his triumph might be witnessed by the human masses. As though he celebrated the damage done to Metropolis, took pleasure in number of innocent lives lost that day. As though his sole desire was and is to be worshiped as a god among men.
But they know nothing about him, could not begin to fathom his life beyond the alien DNA. On any given day he is just as much Clark Kent as Kal-El. He's as in love with the rich and fertile soil as the vast and beautiful sky, yet he has no memory of Krypton save the fleeting images of his parents and a blood red sky. Earth is the only home he truly knows, Martha the only mother who chased his fears away, Jonathan the only father who'd taught him what it means to be a man.
He was not untouched by the fall of Metropolis. He mourns every life marked by the names etched into the memorial; some he knew personally although there were many he did not and now never will. He's no stranger to the nightmares, even months later still looks at the remains of the ship with dread in his heart and unease clawing its way up his throat. What pains him more is feeling that it was all his fault despite the days he's spent telling himself and being told that he was not the cause, that it was Zod and Zod alone.
So he's not a citizen of Earth by choice or genetic code, but this is his world. This is his home, and while he did not chose Earth as his destination, he chooses its people as his people, their hopes and struggles, their dreams and nightmares as his own. He can leave at any time but he won't. He won't because he's grown roots. Because he has a family and a woman he loves far more than any yellow sun. These are his people. This is his world.
With Lois beside him and the creature before him, he makes this declaration. The inhabitants of Earth may not choose him, but he chooses them and will die defending them if that is the cost, if that's what it takes to ensure their survival. After all, they have heroes all around them, more than they know. They have Gotham's Bat and this wondrous woman, and he knows there are more out there committed to mankind, devoted to justice. Should he perish here and now they will continue on in his place.
He moves forward, grasps the spear, flies straight at the beast with every ounce of energy he can muster in the sun's absence.
She is my world. This is her world. This is my world. This is my world. This is my world.
He repeats it to himself over and over as the spearhead devours his strength. He believes it even as further injures himself just to drive the spear that much deeper into the monster that would destroy everything Superman strove to protect. He knows it even as he lets go. This is my world.