i have this story i just started, it is about a serial killer that will be known by the readers but not by the detectives searching for him.
he will eventually be killed but then, it will be discovered that he wasn't able to act alone. another round of investigations will begin and it will be discovered that it was one of the detectives originally working the case that connived with him.
what i want to know is this;

1) is it realistic.

2) won't it be a killjoy, a cop working with a killer?

Write your story first and we'll see, any scenario's possible, it's how you tell it that matters

I think most things are possible in fiction. You just have to make sense.

It is a brilliant scenario! - but one that will require a complex positioning of the character in a way that keeps your target audience in suspense. you might want to research some psychological cases on Dissociative identity disorder (Split personality Syndrome). I am assuming He is a brilliant and consummate detective and also a devious and diabolical villian. You might want to watch Righteous Kill by Al Pacino and Robert de niro where Pacino is a brillant cop who is also a serial killer. Also see The Usual Suspects by Bryan Singer. Infact try to get your hands on Psychological Thrillers.

It is a very challenging project from the sound of it but if you can get the main and supporting characters right, the plot is really a simple "Serial Killer Investigation"

Good Luck

@voltron is so right. You might wanna add Angelina Jolie's Taking Lives to that list.