At the end of my book, the twist is that the main character gives into her selfishness/hate/pride and sacrifices her best friend to a demon in order to bring her brother back to life. The way I have it set up makes it better than it sounds just by generalizing it, haha, but anyway...
The story of the first book ends a couple of chapters after my MC's friend is sacrificed, then the second book begins a couple of months after the fact, with my MC finally returning home with her living brother, feeling guilty and dirty as heck, as you might imagine. Everything is gloomy and dark for her, she just got told that she was an evil b*tch by the knight who romanced her friend, the same guy who also proceeded to abandon her (his new friend whom he believed he could trust) to a group of fanatic witch doctors in blind rage, but everyone in the kingdom is celebrating because they have their king (her brother) back, awesome. To top it all off, she still hasn't resolved her bulimia problem. In fact, that goal seems impossibly far after what she did, or so she feels.
She resolved getting her brother back in book one, but didn't quite manage to gather enough evidence to amass a revolution against her father's murderer (who took his place as king until the princess managed to bring back her brother, whose death was also caused by this usurper), so that's one of her goals in book two. But along with that is what has now become her main goal. Given that she wants to get rid of her guilt and really misses her best friend, the princess sets to studying a ton of forgotten magics to figure out how to resurrect her after she lies to her friend's parents, telling them that their daughter willingly gave her life for the good of her home and that she was a true heroine.
So my question(s) is this, should my main character succeed? Does her friend actually returning to life invalidate that awesome twist at the end of book one too much? Should she give something in return to save her? Is it stupid of me to make the readers love the friend throughout all of book one and then have her die and never show up in the series again? Would
it be way too dark and tragic for her to stay dead knowing that I intended the series to be about hope for a better future and seeing the good in the midst of all the muck we have?
The challenge I see is that your main character loses the moral high ground by sacrificing an innocent person, a friend even, for the sake of the selfish goal of wanting her brother back. Readers have a harder time identifying with an immoral character (and her action would be seen by many as immoral).
Not that you can't tell a story from the point of view of a villain, but such a choice creates a darker tone more often found in horror or suspense stories. It distances the reader from the main character. (Everyone likes to see themselves as moral, so they don't identify with immoral characters.)
A couple of possibilities if you want to your reader identify with and admire your main character...
1. Make the friend sacrifice herself. Rather like the death of Spock in the Star Trek films, a selfless act would keep the friend and the main character on the moral high ground.
2. Foreshadow the existence of a way to bring the dead back to life, so you can later reveal that the heroine had it in mind at the time of the sacrifice. When faced with the impossible choice of being able to save only one of two innocent characters, the real heroine finds a way to save both. There's a big difference between sacrificing a friend and seeming to sacrifice a friend while actually planning a way to bring her back. However, it's fun for the readers to temporarily doubt the heroine's goodness, and then have their faith restored later.
The foreshadowing the restoration trick is important so that it doesn't look like you're just pulling a rabbit out of a hat.
You don't want the heroine to sacrifice her friend and then later think of a way to bring her back, because that still makes the sacrifice immoral. You want the reader know, by the end of the story, that the heroine intended to bring the friend back all along, even if that is not clear at the time of the sacrifice.
3. Make the friend's death an accident. Again, this keeps the heroine on the moral high ground.
Best of luck.