Trapped in an Office Building at the End of the World, I Have a Buffet System

Before the end of the world, he was a "good old" programmer who was bullied and resigned. He was ruthlessly abandoned by his former colleagues and boss, and starved to death in the utility room. After the apocalypse, he returned with the "Buffet Sign-in System" and his character completely changed from cowardly to cold and decisive, with contempt for the perpetrators and the ultimate pleasure of revenge in his bones...