Here are the Supernatural episodes that aren't especially... supernatural. Eric Kripke's Supernatural premise is simple but effective - two hunter brothers prove that blood is thicker than water as they battle through a weekly procession of the spooky and biblical. And when it comes to the monsters, Supernatural certainly doesn't disappoint. Season 1 featured vengeful spirits, murderous wendigos, and demons with glowing eyes, and by season 15, the Winchester brothers were facing off against God himself. On the way, they encountered vampires, werewolves, banshees, angels, Norse Gods, shape-shifters, leviathans, Amazonians and everything in between.
A great many horror stories run with the idea that "humans are the real monsters." Whether it be zombie flicks or vampire tales, mankind is so often the root of all evil, whether through greed, cowardice or cruelty. Supernatural never seriously takes that route. As far as Sam and Dean Winchester are concerned, the monsters are the real monsters. Nevertheless, Supernatural featured its fair share of human baddies over the years, from rogue hunters and the British Men of Letters, to treacherous individuals making deals with Hell's finest.
Even so, precious few Supernatural episodes feature only ordinary humans (meaning mortal characters with no magical powers or monster heritage whatsoever) in villainous roles. The first anomalous episode comes in Supernatural season 1 with "The Benders." This innovative and twisted offering finds Sam and Dean captured by the Bender family, who have hunted innocent people as "game" for generations. As messed-up as they might be, the Benders are still human, relying solely on weapons and hunting skills, although the high disappearance rate sparked concerns even from John Winchester. The next monster-free adventure wouldn't come until Supernatural season 4's "Family Remains." Masquerading as a routine haunted house case, the "ghosts" terrorizing the property are eventually revealed to be children born through incest and tortured by their attacker.
Four seasons passed before Supernatural's next human enemy. Season 8's "#THINMAN" puts a non-paranormal spin on the Slenderman online creepypasta, leaving the Winchester brothers and the Ghostfacers convinced something strange is in the neighborhood. The murders are actually being committed by the local sheriff's deputy and a diner busboy, both of whom are sociopaths that feel no remorse for their spree. The Winchesters polish the pair off and swiftly move on. Four seasons later (perhaps it's intentional?) the finale of the British Men of Letters arc turned into a human vs. human affair. Titled "Who We Are," the American hunters strike back against the Brits who plan to kill them, with Sam Winchester leading one final assault on the compound. A few paranormal antagonists are referenced, but only the humans at British Men of Letters HQ appear.
It's testament to Supernatural's storytelling versatility that the show peppers in a handful of human-only stories, but the overall focus remains firmly on the extraordinary and unusual. Many of the above episodes exploit audience expectations that Supernatural villains will be monsters, genuinely surprising viewers when humans are revealed to be the real culprits. But Supernatural wisely ensures such installments are few and far between. Not only does this keep the novelty fresh when humans do appear, but the scarcity of human villains also prevents Supernatural veering into Scooby-Doo territory, where every week a paranormal problem proves to have a more ordinary source. Still, they would've gotten away with it if not for those meddling Winchesters.