What was Johns beef with Dutch?

Bingo!

Your right, I forgot about that quick exchange early in the movie.

"You work your side of the street, and I'll work mine"

He could have called the cops to take Danny away, but he enjoyed making his fires too much.

"You work your side of the street, and I'll work mine"

Not meant to offend but how did you draw such a wild conclusion from the film? That contains way too much supposition.

Ben's dad didn't hurt Dee Dee. She was the one having the affair and she was the one that wanted to run away with Butch.

Timeline of events if you listen to all the dialogue:

-Dee Dee is having an affair with Dutch. Separates from husband.
-Ben moves in with John.
-Dee Dee kills herself. Ben's father leaves.
-Dutch has a religious experience and repents his sins. Confesses to John that he was present when Dee Dee killed herself.
-John snaps and attacks Dutch and ends up killing him(intentionally or not). Disposes body.
-Danny realizes John killed Dutch. John kills Danny when it's cleary Danny is going to kill/burn the house down. Either way he knows Danny is not going to let it rest.
-John hesitantly kills Danny, disposes body.

I really am not sure where you got the idea that he's a serial killer. There's no dialogue in the film to suggest this. Where would the justification be to kill Ben's father, and his own sister? And if he killed his own sister over a decade prior, why would he randomly kill Dutch after the fact? And why would Dutch confess to being present during the suicide if she was murdered elsewhere by someone else? Again, too much supposition, made up scenarios not in the script in any capacity, etc.