This needs a little explaining. My book is a fantasy book. It takes place from an elves point of view. Tension between a country of elves and humans are running high, and elven villages have been burned by supposed humans. A group of elves is made up to catch the humans responsible. the main character is in that group, and another of the six in the group is going to betray the group in the end. The characters motives are to instigate a war between the humans and elves as she believes the elves will win, and that it will spur the elves into an economic and militaristic dominance over the dwarves, orcs, and remaining human nations. I was going to have her be kind of a quiet loner type, but still contribute and possibly save a group members life. Either the main character or a love interest. At the end she is going to convince him to come to a burning human village, where the love interest is "locked up". Of course then she will somehow paralyze him, and monologue revealing her motives and the whole reason for betrayal. Any tips?
Mislead the reader by either making the betrayer appear to be actively after someone else with her intended victim's help, or insert a third character who's ire for the victim convinces the reader that the danger comes from them, not the real betrayer. Iain Banks' Inversions uses the first method. But I think any misdirection is a useful method because it adds plot elements that make the eventual twist that much more disconcerting and dramatic.
Subtle foreshadowing is your friend. Not so bold that it gives the betrayal away, but clear enough that somebody can flip back through your book after getting to the twist and say "oh, so that's what that was about". In this case, I'd say you want to have her express these elf supremacist views earlier on. Not ranting and raving or anything like that, but clear expressions of contempt toward non-elves.
