“Calling Dean pathetic and unthinking and being John’s “good little soldier” just following orders….And then SHOOTING HIM, TWICE???
It went too far.”
I guess the writers thought the ghost possession explanation was sufficient. I don’t think the ghost made anyone angry or murderous, but it simply amplified anger that was already there, eventually driving someone to commit murder. The cop at the beginning of the episode had been arguing with his wife, but he’d never have killed her if he hadn’t been possessed. I guess the writers were trying to subtly show us that Sam was already resentful of Dean, and being possessed by crazy Doc Ellicott is what drove him to try to commit murder. My point being that if Sam hadn’t been so perpetually angry to begin with, the ghost wouldn’t have had anything to feed on.
I agree with you that I can’t recall a time when Sam, in his right mind, would’ve intentionally shot Dean. But since he clearly spent a lot of time being angry with John and Dean, it’s not that much of a stretch for me to think that Sam’s existing emotions gave the ghost a big ole buffet of negativity to feed on.