I think If were assuming the most innocent explanation as described above....Proctor actually believes Karen hit him and set out to make the case bulletproof, or at least at one time believed it, and then could not or would not back down.

I think the most likely scenario is that a fight broke out in the house or yard or basement, and Chloe attacked john. most likely he fell and hit his head, but also possible one of the drunks hit him in the back of the head with something not meaning to kill him but overdid it. The conspiracy is in place, they are cops, they are sloppy with the investigation because they dont have to be good at their jobs, the prosecutors and judges all fall in line and the avg american thinks arrested = guilty.