The Hero Wants To Die summary:

At 19, Kim Gunho is transported to the 23rd century with enhanced abilities. After spending 50 painful years preventing an apocalypse, he returns to his younger self at 19. Now, with the threat gone and no one aware of his suffering, all he desires is to rest and even wishes to die. It will take 100 days to fully eliminate his enhanced cells. As he prepares to spend his final days in peace, he is unexpectedly visited by Ye Haejung, the boy he secretly loved before his time travel.