Is this going to be a serious movie or is it going to be a comedy?

It's going to be a serious movie

Can't recommend the book enough, it is facinating! Imagine having known someone who became one of the most notorious and sickest serial killers ever.
The book is very well presented and the author makes it clear he was trying to present Dahmer as he knew him, which was by no means perfect, or without flaws, and not as the person everyone eventually knew him to be.
It is this weird mix of friendship and knowing the person was not quite alright.
Derf doesn't spend the whole time basically saying how everyone knew Dahmer would turn out that way and he was insane. If anything, the story is just tragic.
Please read it, you won't be disappointed!