Would you read or let your kids read a serial killer's fictional novel???

As long as there's nothing bad in the content of the story. It doesn't really matter that the serial killer wrote it. I think he's old enough to read it considering he's already 13. In school, they make you read a lot worse.

No reason why your son shouldn't read the story. The content isn't going to warp his mind. The important thing is that your son wants to read so you should encourage him. After he finishes the novel the two of you could have a discussion about the story, its theme, plot, character, whether it was a well written story or just another cliché ridden one.

When I was 15 I read "Lolita" and it certainly didn't change my life.

No, but considering you've already told your kid about it and wet his appetite there's no point saying no. He'd just go find it himself. Are you sure you aren't 13?

Not my young kids, no. The older ones read what they want.

But they are my kids, not yours. Make your decisions about your children from who you are as a parent.

No, I won't ever do that.