When you’re writing a mystery, especially a classic murder mystery with a proper sneaky villain, one of the hardest things to do is keep the reader guessing. You don’t want your villain to be obviously evil, so the natural inclination is to make them appear as squeaky clean as possible. Of course, a savvy reader will instantly suspect the character who’s always helpful and nice. It’s a tough line to ride.
Dimitri Vorontzov coins the slightly silly term of “green herring” to suggest some solutions to this conundrum. It mostly comes down to treating your villain as a real character with flaws, good qualities, goals and conflicts. Even if some of those things turn out to be clever deceptions when the villain is revealed.
So, what’s more plausible than “a very good person?” That’s right: Essentially good, but flawed, imperfect person.
We can let such a character make dumb mistakes (which we may later reveal to be deliberate acts of sabotage); we can make him or her slightly selfish, or slightly dishonest (a tiny instance of dishonesty may prove their overall integrity); we can give that character some of the “seven deadly sins,” for example sloth or greed.
Anger works particularly well to prove the green herring character’s essential goodness, because when a good character is a little nasty, has a bit of an attitude problem—this sub-communicates that such a character is not hiding anything, not trying to come off as “nice.”