Uchiru's Manga

This is definitely interesting. MC is from earth got reincarnated in another world where fighting demons was neverending and became a knight king and chosen by the Gods as their last hope. Only MC survive after fighting for hundred of years and he returned to earth and propagate his religion. Although story is definitely amazing i dropped it since I confirmed polygamy ending even though romance is almost non existent.

This is about a student who got isekaied and became a hero in order to save his own world. He endured countless hardships and constant life and death situations hoping that his family and friends were waiting for him. After gaining fame and glory his still insisted on coming back to earth but what saw were disappointments. His parents died, his relatives blamed him for their death due to him missing, and his friends abandoned him. His mind broke and became insane massacring everyone due to the unfairness of the world.
One of his victims also got isekaied and returned as a hero who hunts other heroes