Ososugita Isekai Tensei - Jinrui wo Horoboshita Maou desu kedo Yokattara Uchi de Hatarakimasen ka?

One day, mercenary extraordinaire Masamune was reincarnated to another world as a hero who would save humanity–HOWEVER! "The human race has already been destroyed." The one waiting for the man whose reincarnation came too late was Shishi, the demon lord who had already done away with humanity! To Masamune, the hero who found himself out of a job on his very first day, Shishi makes an offer: "Would you like to come work for me?" With nowhere to go, Masamune accepts. Only… "So, who do you want me to assassinate?" "Uh, no!? Nobody's asking that of you, okay!?" With the demon lord's castle as the stage, this former hero sprints towards his future at top speed!! The curtain rises on this sometimes love comedy, other times great otherworld fantasy!!