Serial killer is a term generally used to describe someone who commits three or more murders at different times, and usually committed in different places. Ther term is used in this sense in supernatural in relation to humans, although many vengeful spirits and demons would also fit this definition. In fact Bela even refers to Sam and dean as serial killers.
"drifting serial killer"
1.07 Hook Man: in 1862, Jacob Karn killed 13 women. His spirit continues to kills people, feeding off repressed emotions.
1.12 Faith: David Berkowitz is the name on the credit card that Sam gives to the nurse in the hospital to pay for Dean's treatment. There is an extremely famous person with the same surname - David Berkowitz, the serial killer also known as Son of Sam.
2.05 Simon Said: Dean comments when examining Andy's van "Not exactly a serial killer's lair, though. There's no... clown paintings on the walls, or scissors stuck in victims' photos. I like the tiger."
2.06 No Exit: H.H. Holmes is often called America's first serial killer. The number of his victims has typically been estimated between 20 to 100, and even as high as 230, however, the only verified number is 2. In 1893 he opened a hotel for the Chicago World's Fair and used it to trap, torture and kill his victims. in this episode, his ghost is haunting the apartment building which was erected over the site where he was hung. When the Winchesters are researching people killed at Moyamensing prison, it is Sam who recognises that Herman Webster Mudgett was H.H. Holmes real name.
3.15 Time Is on My Side: Doc Benton found the secret to immortality and over many years has killed people for their body parts to keep himself alive. 7.15 Repo Man: Jeffrey was a postal worker with psychopathic tendencies, who was possessed by a demon who started killing people. After the demon was exorcised, Jeffrey eventually began killing on his own.
7.19 Of Grave Importance: Whitman Van Ness killed his fiance, and a number of other women. He died at the age of 40, and continued to haunt the Van Ness House, killing many people over the decades.