Yongsa-ga Dorawatda

After saving one world, warrior Min-Su Kim returns to Earth only to find that he’s lost everything... so he decides to destroy the world he used to call home. In his rampage, he kills teenager Jeong-Su Park and his family. But instead of dying normally, Jeong-Su is transported to another world, where he finds that he's now a warrior himself. After killing the demon king of this alternate world and honing his skills as a warrior, Jeong-Su returns to earth, hell-bent on revenge against Min-Su. As the two warriors fight with the fate of their country in the balance, who will emerge victorious?

(Source: WEBTOON, edited)