Vengeful spirits was a term employed to refer to the disembodied spirits of deceased humans who demands vengeance.
Most of them were murdered under horrible circumstances.
Known Vengeful spirits
Sid was a Demon hunter in the 1930's. At some point, he was attacked by the Brotherhood of Seven, a group of demons who were able to assume human form for seven years at a time by harvesting human brains and hearts. The Brotherhood cursed him, imprisoning his spirit in a simple ventriloquist's dummy. The only way for Sid to break the curse was to destroy all seven members of the Brotherhood.
By the spring of 1997, Sid had succeeded in killing all but one of the Brotherhood.
Maude Pearson was believed, in twists, that she was murdered by his son, Dennis. It was revealed that it was the opposite. Not supporting that he leaves with his girlfriend, Dennis was locked living behind a wall by her mother before she dies from a heart attack. Her ghost haunted the appartment, harassing and growing Cordelia Chase to the suicide. This last broke down the wall, it unleashed the spirit of Dennis, who destroyed his mother's ghost.
Hus was a Native American spirit warrior of the Chumash tribe which returned to exact vengeance against those who wronged his people.
His spirit was somehow bound within an old Sunnydale Mission which was buried by a huge earthquake in 1812. Everyone assumed the mission had been leveled by the earthquake, and the town unknowingly built over it. He was, accidentally woken, by Xander Harris, then used as worker to make excavations and wants to kill the major warriors of Sunnydale like the Slayer.
Katrina appeared as a spirit summoned by Willow Rosenberg when she was Dark Willow to torment Warren Mears after she realized that Tara Maclay was not his first victim. Katrina was understandably angry with Warren, and suggested that she should have killed him before he killed her. It's unknown if this was an actual ghost or illusion created by Willow.
These three Vengeful spirits haunted the newly re-opened Sunnydale High School.Persuading that they were people whom the Slayer did not have was able to save, they turned out to be just illusions generated by a talisman, apparently put by an agent of the First Evil.
Matthias Pavayne was an 18th century European aristocrat and a doctor that was nicknamed the Reaper for performing unnecessary, often fatal, surgeries on his patients. When word spread of his unorthodox practices, he escaped to California. His arrival coincided with a rash of brutal, ritualistic murders; pieces of the victims placed in such a manner that suggested an intimate knowledge of the dark arts. The murders continued for nearly twenty years until 1791 when representatives from Wolfram & Hart were looking to build a new branch in what would eventually become Los Angeles. Unfortunately, a Spanish mission was already resting on the site their seers recommended. They needed an appropriate sacrifice to deconsecrate the ground. Pavayne's blood suited the needs of the situation and Pavayne was killed by Wolfram & Hart. He was accidently woken by Spike then immaterial.
The Hermanos Número were a group of five Mexican wrestlers brothers who lived at Los Angeles during the 1940's, protecting the chicano community from the evil that plagued Los Angeles, whether human or supernatural. Four of them was killed by the Aztec demon warrior Tezcatcatl. By 2003, the last brother Numero Cinco conjure a ritual who summoned the spirits of her brothers for to eliminate the demon with Angel's help. They had the particularity to be materialized, similar to zombies and a limited alteration power. For example, when conjured, they left their grave like zombies or vampires; they return to their grave as if it had never been opened.
Most have the capacity to make material or to possess an object as a doll, often to kill.
Contrary to the other spirits, Sid and Hus are not being allocated that in a single place and has the peculiarity to move everywhere where they want.