The Mutual Non-Aggression Is Unpalatable summary:

"With the memories of a modern man, Ja Hyeonghwi was reincarnated in a different world as the legitimate son of the Imperial Family, Hyeonwol. While he became a picturesque emperor, led the country to peace, and lived peaceful days, he was killed by the protagonist of this world whom he did not even know existed - the Heavenly Demon Seol Cheon-yu.While regretting realizing too late that the reincarnated world was a novel, I thought the supporting role's fate would end just like that.... When he opened his eyes, Hyeonwol was still the crown prince and he returned to his 20-year-old body. When the second chance presented itself, in order not to be killed by the protagonist in this life, I decided to be completely on the protagonist's side.However, aside from the fact that the path chosen to live a comfortable life faltered from the start due to the aftereffects of death, the protagonist's attitude upon their reunion was quite odd."+