Maybe Kotsume figured out he missed the underworld too much to leave...and that makes him sad because he has to choose between the underworld and Lord Poseidon? ╮( ̄▽ ̄)╭ Not a spoiler, as I have no idea what is coming next, just brainstorming reasons for why he is crying at the end there.
I'm not really sure but I feel like he would go back to do the underworld....simply because he has become so accustomed to the place now all his hard work and memory is of the underworld....
Maybe Kotsume figured out he missed the underworld too much to leave...and that makes him sad because he has to choose between the underworld and Lord Poseidon? ╮( ̄▽ ̄)╭
Not a spoiler, as I have no idea what is coming next, just brainstorming reasons for why he is crying at the end there.