Killer is Dead: Nightmare Edition is an action game set in a futuristic world where players take on the role of Mondo Zap, a contract executioner for the Bryan Execution Firm. The game is divided into 12 episodes, each set in various global locations, and features a rich narrative interspersed with animated sequences that enhance the storyline. Players engage in combat against cyborgs and assassins, wielding a katana as their primary weapon, which is a hallmark of the developer's style.

The gameplay is structured around main missions that, upon completion, unlock various side missions, providing players with additional challenges and opportunities to explore the game's universe. The cel-shaded visual style is distinct and contributes to the game's overall aesthetic, making it visually appealing while maintaining the intense action that fans of the genre expect.