Everyone lives. Beaten bloody and broken, perhaps. They will need much patience and a long time to heal. But they made, and they do heal.

Justice is served. Most victims or unfortunate participants make it through the story. They lick their wounds, mourn the dead, and testify in court or deliver justice in whatever way is available to them.

Supernatural ways to fix things or enact justice are allowed so long as the characters earn them somehow and they don't just come in to twist the ending.

(Actually, even that type is fine. Just tag it clearly as having a bit of a deus-ex-machina.)

That's a hard one, there aren't a lot of horror manga to begin with. You've probably read it already, but I'd say Tokyo Ghoul fits. And if I remember (it's been a while) Deadman Wonderland does as well.