Bittersweet as all hell and can leave you feeling hollow like with the anime ending but it's at least not "haha they elope and have happilyeverafter pedophilic relationship" or "everyone dies lol togetherindeath". She's screwed up but in a way that Sato can't really get at her anymore, even if she's left her mark - ditching the Together in Death romanticism to do what was actually best for her was like half a redemption I suppose.
I'd like to see more of older(teen?) Shio and how the cast get on a decade later, there's certainly enough to write about with all that's happened. Wonder what the manga will do next.