I suppose, if the show wanted to annoy me even more, they could make the final conflict/resolution about quibbling over the BMoL's precise means instead of the full implications of their ends. Reveal that they've been assassinating humans and have that be where the Winchesters draw the line.

I know this could never happen on the show, but I have been entertaining the premise: what if that wannabe Jack Bauer dude from 12x09 actually managed to track/put Sam and Dean - and by extension the BMoL - under surveillance and the US intelligence services are about to blow the roof off everybody's operation?The big bad hell our heroes unleash this season turns out to be the US government and the lengths they (and the American public in a monsters-are-real frenzy) would be willing to go to in the name of, er, securing our metaphysical borders. Which would mean a dystopian hellscape brought on not by monsters but by the people who fear them.