The person is changed from a human into something that eats humans. It does bring up a difficult question about whether we can call a different species "evil" for going along with its nature but humans aren't going to like it.

@g33k,

While I may have misremembered (I can't find the specific WOJ), I don't think it's improbable that Harry would have been able to do a mini-Darkhallow. I think that Harry's been tempted to do something like that several times from the very beginning. I'll add a description of the very first time that this has happened in the end of Storm Front below here. For context, Harry has just driven up to Victor's house and he's thinking about his past brush with Dark Magic, the suspicions he's gotten from the White Council, and how slighted he feels. 

[Harry] could kill the Shadowman, now, before he knew I was here. I could call down fury and flame on the house and kill everyone in it, not leave one stone upon another. I could reach out and embrace the dark energy he had gathered in this place, draw it in and use it for whatever I wanted, and the consequences be damned.

When I look at the part that I italicized for emphasis, Harry is pretty clearly thinking about a Darkhallow-type action. While he isn't discussing absorbing the power of ghosts permanently, he is pretty clearly discussing using the dark emotional energy to power a destructive spell. Harry has also made it clear at other points in the series that doing things like this affects the practitioner. I think that this is what evil Harry did in the end of Grave Peril. Evil Harry absorbed the emotional energy of Bianca's building then lashed out at Bianca with it. Our Harry summoned ghosts then pointed them at Bianca. My memory isn't perfect though so I might have messed something up.

Monica participates in the orgy rituals that Victor used to create them. I think she's somewhat culpable but minimally so. Especially compared to Victor and the married couple (I forget their name). There's also a pretty good argument suggesting that Victor manipulated or abused her into doing it so she isn't.

Jim has directly addressed this if I remember correctly. Harry asked for and got the ability to save Maggie. He gets strength as the Winter Knight, Lea's temporary help to get Maggie out, and the continuing ability to walk. If Harry asked for a salary, I could Mab saying something "To people paralyzed like you actually are, the ability to walk would be priceless. This is your salary."

I agree. I think that the best evidence would be a continuity error in Peace Talks and Battle Ground. They were originally written as one book before coming out as a two-parter.

My suspicion is that the Chitchen Itza event was actually required to start bringing over the Outsiders to help the Formor. A fairly consistent part of the magic system is that the mood or atmosphere of the world matters. Dead Beat had Cowl messing with Chicago to "prime the pump" and Battle Ground showed that the fear of Chicago affected the spells cast during the battle. Since the big spell at the end of Changes affected so many people who weren't the Red Court, I suspect that certain spells would have been easier in the wake of the bloodline curse. I bet that it would have helped summoning an Outsider that is described as a peer to Mab. After being summoned, the Walker then helps the Formor assemble their forces and/or knock down some one like the king of the Tuatha as payment. It then starts assembling it's forces to assault Demonreach.

As for what the Outsiders get out it, beyond a Walker that gets summoned, is more chaos. Mab, Odin, and Uriel are chief forces in alliance for the current order among the natural world. Mab and Odin also established the Accords. Eliminating the Accords could pull away some of Winter's forces from the Wall and a massive global war would kill a lot of the people who could stand up to the Outsiders after they breached the Wall.

I hope the fact that Lara and Harry share a brother, but aren't technically related, is a source of joking or tension in 12 Months. Lara grew up when the Hapsburg chin was still a good thing and she's had way more incestuous sex than with her half-brother's half-brother (thanks Papa Raith). She almost certainly won't be bothered by this relationship to Harry but he probably will.