Nature? Divine. Human nature – toxic.
Humans are the dominant species of Earth, gifted the soul by God some point after coming into being. Humans are considered God's greatest creation, however their favor would draw the ire of Lucifer, who saw them as flawed and murderous things. The view of humanity among the rest of the Host of Heaven varied, some like Castiel saw humans as works of art, while others like Uriel saw them as lower beings, referring to them as "mud monkeys" and "savages" and comparing them plumbing on two legs, beliefs Castiel warned were close to blasphemy.
Throughout the years, Sam and Dean have encountered threats they believed to be supernatural, only to find that they were dealing with a completely human culprit. A fact which Dean frequently would comment on, revealing his disgust at the lengths people will go for to hurt others, calling them "sick" and "crazy", seemingly preferring the simplicity of actual demons and monsters.
The Winchesters first encounter with a human threat were the Bender family, consisting of patriarch Pa Bender, brothers Jared and Lee Bender and the youngest -- Missy Bender in Hibbing, Minnesota. The Benders would kidnap a man by the name of Alvin Jenkins while taking out the trash on night, leading to Sam and Dean arriving in town to investigate the disappearance, due to John marking the area as a possible hunting ground for a phantom attacker based on local folklore.
When Sam gets abducted, Dean teams up with a local deputy, Kathleen Hudak, whose brother went missing three years prior under the same mysterious fashion. When Alvin is set free, he attempts to escape, to Sam's protests that it a trap, but is caught and killed by Jared and Lee. After finding the Benders home and locating Sam, Dean goes to find the key to free him, but is captured soon upon entering the Bender home. While restrained, he learns that the Benders hunt humans, but plan to kill Sam in his cage. When Sam gets free with the help of Deputy Hudak, they take out Lee. When Pa and Jared go to check on him, they are able to incapacitate both men, while Dean freed himself and locked Missy in a closet.
In the aftermath, Deputy Hudak confronts Pa about her brother and asks why they do what they do, only to get the cackling response, "because it's fun." Horrified Deputy Hudak takes her gun and shoots Pa, killing him.