I'll Join the Demon King's Army for Revenge

After choosing to take his own life and donate his heart to his twin sister, Ito Xiao comes back to life in the world of a game. This time, he is the one his sister protects. After a series of events, Ito Xiao curses his own powerlessness and joins the Demon King's army to gain strength, slaughtering players in the Hero's army. (Dark story)