My guess is that there really wasn't an explanation. Or rather, I agree with you about it being pretty obvious/unimportant- either the spirit was just that pissed off, or it's a product of Dean's character, or it's just the kind of thing that would apply to pretty much any hunter (the link between the secret and someone's death).

My guess is what Kripke really meant was something like, "If we make something up in the future, we can throw a reference back to this and it will look like clever continuity."