And that's the good ending! Omori defeated the murderer that took Mari from everyone and rid them of this world, the people who thought Sunny was their friends were reunited and will be together again, Basil's probably not going to be okay but he was complicit in the evil so whatever, and Omori gets to live with his friends and sister happily ever after. The End.
I think the understated part of this ending, at least to me, is why Sunny does it. It's because he's a coward. What Omori says is what he believes. He's not killing himself just because he can't live with what he's done, but also so he doesn't have to tell his friends what he did. He's afraid they might hate him. That he'll lose them. It's why there's no kind of middle ground ending where he tells them and then does it. He kills himself so he doesn't have to tell them the truth. Basil definitely won't after this, and is probably going to follow him eventually. Hero, Kel, Aubrey, they'll never know what he did. They'll still love him. They'll never learn the truth and be left with thinking they lost half their childhood friend group to suicide. And Sunny won't have to live with what he's done. Killing Mari, lying to them, telling them this, any of it. He won't have to face the consequences of his actions. Sunny kills himself because he's selfish.