The Footsteps of Cain by Derek Kohlhagen
He made a mistake. His punishment? Kill the world.
A nameless immortal wanders a desert of his own making, on a planet mostly devoid of life. He is imprisoned to the will of a malevolent spirit...limping toward atonement through a chain of uncountable bloody deeds, and across many millennia. The only thing that keeps him going is the promise of freedom...not for himself, but for a love that he has destroyed.
His ages-long road has led him to the distant future, to the planet's very last settlement of humanity. There, with one final, tired massacre, he aims to end the world.
He will find the last remnant of civilization unwilling to enter the void so easily. He will also find that the world has one final, soul-stirring revelation yet left for him.