So there's this academic Dannagal G Young, who studies liberal and conservative news media, and why they have such different aesthetics. (her book on the topic)
One of the key differences between liberal and conservative media is liberals have a much strong appetite for satire, irony, and humor, while conservatives have a much stronger appetite for outrage-inducing content.
Young (who is a psychologist/social scientist by training) argues that one of the key psychological traits that commonly divides liberal and conservative people is liberal people are typically okay with and even enjoy ambiguity to a much higher degree than conservatives, while conservatives crave clarity and closure, and see the world as inherently dangerous and threatening, in a way liberals typically don't. (This pattern has been affirmed by a great deal of social psychology)
From her TEDtalk on this topic (starts at roughly 4:00):
"Because they [Liberals] are tolerant of ambiguity, they also tend to be OK with nuance, and they enjoy thinking for the sake of thinking. Which helps explain why it is that there are distinct aesthetic preferences on the left and the right, with liberals more likely than conservatives to appreciate things like abstract art, or even stories that lack a clear ending...these differences help explain why ironic political satire is more likely to be appreciated and understood by liberals than conservatives. On the other hand, those people who are monitoring for threats, who prefer certainty and closure, those tend to be our political, social, cultural conservatives....And we've found that these traits help explain why conservatives enjoy political opinion talk programming that clearly and efficiently identifies threats and enemies."
Hmmmmm....let's think why Supernatural might the most perfect blend of these liberal and conservative aesthetics.
This tolerance for ambiguity, and enjoyment in thinking for the sake of thinking is a trait that makes one very predisposed to key into things like symbolism, subtext, textual paralleling, foreshadowing and call-backs, literary references, and so on in media. In my life, I have never encountered a show with as much of this type of content as SPN has.
Whereas conservatives tend to key into media at the most surface level and to respond strongly to storytelling like police procedurals where there is a distinct threat that is identified, and ultimately eliminated, with a satisfying, clear conclusion. The MOTW structure is perfectly designed to speak to that element of conservative aesthetic taste.
But while conservatives watch Supernatural and think the "white men shooting people they deem less than human and facing no consequences for it" is all there is to the show, liberals take that aspect of it and read it in conjunction with the show's subtext, symbolism and so on.
If you asked me what Supernatural was 'about' in a sentence or two, I would say it was commentary on the unique structure and anxieties of violent conventional white American masculinity.
The question with Supernatural was always - is this a deconstruction of/commentary on violent white American masculinity, or is it just an unself-aware reiteration of it? From a literary point of view, the intense homoeroticism acted as the most clear fulcrum on which one would eventually discover if conventional white American masculinity was ultimately being upheld or deconstructed here.
(In key Supernatural fashion, we never really found out because Dean never is 'allowed' to address his feelings for Cas post-confession. Which I guess means no...)
But anyway, I think it is useful to think about Supernatural's appeal across the liberal/conservative divide within this larger body of academic work. It illuminates a lot, I think.