the devil wears pink
Dahlia Hawthorne: killer, temptress, and apparent lover of all things pink. As far as final bosses go, she's certainly unique in her appearance, but behind that sweet smile is a heartless demon. Mia has the misfortune to go up against her in her very first trial with tragic results, but eventually gets the best of her at the cost of losing Diego.
Mia
As I wrote on the revenge page, I think putting Dahlia behind bars was more about closure than revenge for Mia. What I'd rather focus on is Mia's relationship with Dahlia. The player (along with Phoenix) learns in the last case that Dahlia is actually Pearl's half-sister and therefore Mia's cousin. But did Mia know this?
It's possible. Mia and Diego fully investigated the kidnapping case, looking for some way to get Dahlia. Her past, even with her little disappearing act, would not be too difficult to find out for Mia, considering her family line. But Mia never mentions anything about it at Phoenix's trial (from the writers' standpoint, probably for the dramatic effect it would have later on). So, it's a possibility, but it's uncertain.
Dahlia, on the other hand, surely knew about it. Mia's name, coupled with the magatama she wears, was a dead giveaway to Dahlia, who reacts when she first sees her in court. After Dahlia's lies are uncovered, Dahlia pointedly tells Mia to prove it, using a far different tone than with everyone else present — all caps, the same way she does in the final case. It's a fair bet to say that Dahlia knew, and couldn't let it go without a pointed mention.
Diego
For Diego, it is about revenge. After all, Dahlia is the one who put him in the coma that stole five years of his life away and took him away from Mia. He knows, of course, that Dahlia has been put behind bars and even put to death, but he goes far beyond the extra mile and keeps tabs on her plans even in jail so that he can stop her from killing Maya. When he finally does see her again, it doesn't matter to him that she's being channeled by either his accomplice or a nine-year-old girl.
But I still picked up the blade... It was like I was dreaming! I'm not sure exactly what was going on in my mind at that point... Was I really motivated by the pure desire to protect Maya Fey? Or was it something else...? Was it my hatred for a woman who had stolen everything from me 6 years earlier...? Could it have been simply a desire for revenge?
It's in that moment that he forgets everything that Mia accomplished in life: that she did succeed and put Dahlia behind bars. He does what he went there to do, of course: to protect Maya from Dahlia. But he doesn't think about what he's doing or the consequences; instead, he's consumed by revenge. And this revenge ends up costing him his own life in that once it's discovered, things can never go back to the way they once were.