It's nice to see him kill some sick ass people and proceed to do the same again and again , kills all of the world and end himself . Wouldn't that be enough to finish this off
Having read the volume 1 of the LN, I can say with absolute certainty.
They deserve it. All of them. From nobles to peasants. Holy shit, I can't believe how disgusting everyone with a hand in the hero's misfortune is. From this knight guy to his wife's family, the ones who worked for them, the peasant village nearby, and especially the stupid princess and her knight.
What he's going to do might look like some contrived, edgy revenge, but really he's just paying them back. This knight guy for example.
He's the reason why the hero was a orphan in the first place. King ordered him to raze his village and kill his family, so that they could blame it on the demon lord army, and he did that, getting that disgusting taste from human flesh from eating the hero's mother and little brother. So, naturally the hero.
Trashed him in combat until he was left a sobbing mess, then made him eat his wife, made his firstborn eat his brother, and then forced him to eat his firstborn.