Castle is a murderer no question, a serial one two. Which is why I think it goes back to him working better as a character who plays off others. He's somewhere between an anti-hero and a straight up villain. Back to my Rorschach example people love that character but he worked best playing off the other less murder inclined Watchmen. It's still hard to picture a whole series so focused on Frank and his warped mentality where he will be the protagonist.