David Berkowitz

"Sam's Creation .44" or as newspapers dubbed the "Son of Sam," Berkowitz, a paranoid schizophrenic and former U.S. Postal Service employee with an above agerage IQ, claimed that voices were telling him to kill. In the end, he had killed six people, blinded one, paralyzed one, and wounded an additional seven people - his targets were women with long, flowing hair.
The thing about Berkowitz was that he was so random in his choosings. His "blood-thirsty demons" that tormented him required that he killed, but they also provided an illusory shield around him making him invisible to police because they couldn't catch him. At the time of his capture, Berkowitz told police that he was carrying out the wishes of Sam, a 6,000 year old demon who passed his instructions to him through his neighbors black Labrador retriever. He claimed, "Without Sam, I'm nothing," because he was pretty much a nobody who had no achievements, no friends, and no attachments. He was found able to withstand trial, declared legally sane, and pleaded guilty.
In February 1979, he called a press conference to let people know that his story about demons was just a fabrication that was "invented by me in my own mind to condone what I was doing," but most psychiatrists tend not to believe his claims that he was not delusional at the time. For example, Daniel Schwartz and Richard Weidenbacher claim that this denial of demons may be because Berkowitz spent time with a Baptist minister who convinced Berkowitz that he could only accept divine forgiveness if he took full responsibility for his crimes, and so, for the sake of his soul, he renounced the demons even though they were quite real to him.
There is also a theory that Berkowitz was a member of a cult known as Process Church of the Final Judgment, which is theorized to have been influenced by Scientology.
For more information, visit the ABC news archive containing the Eyewitness News Exclusive on the Son of Sam. In this interview, Berkowitz mentions The Process as the cult in which he participated, which met at Untermeyer Park, in Yonkers NY. The interview also gives the details of the alledged conspiracy that caused the Son of Sam murders. For an interesting theory of Berkowitz's "play on words" in his Satanic cult, visit Thee Underground's page.
Religious Tolerance.Org gives an explanation why this is unlikely. They claim that Berkowitz mentioned that Sam was really based on the Celtic god of the dead, Samhain (which he pronounced "Sam-hane"). They also mention that the Celts do not have a god named Samhain, and also that Satanists do not worship Celtic deities. Samhain (pronounced "Sah-win") actually means "end of summer," and remains a Wiccan holiday.
You can read the Process' Satan on War propaganda.