Warning: Spoilers ahead.
Mernes asks what Leo really wants. He can tell Leo doesn’t really want to destroy humanity. Leo tries to change the subject but it’s no use. Mernes wants a straight answer.
But it does lead Leo to learning more about Mernes. Mernes actually did want to destroy humanity because he was half human half demon. His childhood was terrible. He suffered a lot. He somehow survived as an assassin.
Then Echidna began her invasion of the human world. He thought she would destroy everyone. So he used his spy abilities to secretly help her take over a town, without her knowing.
Turns out Echidna did not plan to ruthlessly kill the town’s inhabitants. Instead, they imprisoned the town’s corrupt mayor, and shared the mayor’s resources with the humans. In return, the humans helds a feast to show gratitude.
Mernes couldn’t comprehend this. It was ridiculous to him that they were breaking bread together in peace.
Time passed, and Mernes rose to be one of the great generals in Echidna’s army. He secretly planned to take over the army if necessary to complete his goals. But then Leo showed up and defeated him.
That time when Leo first defeated him, Leo gave advice to Mernes. Leo said he should find another perspective other than the miserable assassin life he knew. Travel the world and find something to love, basically. That’s why he can’t imagine Leo actually wanting to destroy humanity/this world.
So Leo finally confesses about his past. Apparently he didn’t isekai travel from the world we know, filled with modern civilization. The world they are in… is the world of modern civilization! One day, monstrous demons arrived out of nowhere and pretty much defeated humanity. Humanity responded by creating 12 people with demon traits called the Demonhart Series. Leo is supposedly the only survivor. Their mission was to protect humanity.
Leo has survived over a thousand years. It’s not very realistic that humanity falls back into the middle ages with no trace of the advanced technology that once was. In just a thousand years. Even 5,000 years. But whatever. So in the end, it’s the demons who are doing the isekai traveling.
He felt maybe humanity had become too dependent on him. And he didn’t feel free. So when humanity actually called him a monster and didn’t want him as a hero anymore, he was relieved. He could live a new life. And in that life, he wanted to help demons accomplish their goal.
He tells Mernes that Echidna's group are actually really good people. He knows this after living thousands of years.
This is a lot for Mernes to take in. He doesn’t know why Leo would tell him all this. And Leo says it’s because he’s been wanting to tell his story. And Mernes is actually good at communicating.