Matt Bergstrom was a serial killer who murdered at least six women in New York County over a five-year period and possibly another woman in Houston, Texas. He also assaulted a prostitute, but was not convicted due to lack of evidence.
History
Detectives Briscoe and Curtis arrested him on suspicion of assaulting and torturing a woman named Catherine Lansing, and uncovered evidence that tied him to several murders. He offered to reveal where he buried his victims in return for the D.A.'s office taking the death penalty off the table. Eventually, however, the detectives discovered Lansing had in fact been assaulted by someone else, and that Bergstrom had played the D.A.'s office for fools. As part of the deal he made with ADAs McCoy and Carmichael, he had immunity for the other murders, and they could not knowingly convict him of a crime he did not commit.
Ultimately, however, McCoy and Carmichael tricked him into believing that they had evidence tying him to a murder committed in Texas, and that they were going to extradite him there, where he would be quickly executed. Panicked, he confessed to the torture-murder of one of his New York victims in exchange for a life sentence without the possibility of parole. (L&O: "Agony")
Psychological Profile
Forensic psychiatrist Dr. Emil Skoda theorized that Bergstrom killed to act out on his violent sexual fantasies, which involved humiliating, degrading and torturing women. He would refine his MO as he went on. He was well-organized and careful. He studied crime-scene procedure. He planned the murders down to the last detail and acted decisively. He was meticulous, organized, and in a line or work where he dealt with minutiae and traveled often, giving him a chance to kill without having to be tied down to one place. He had no prior relationship with his victims. He blended in and appeared normal to others. He also collected sadistic pornography and kept a detailed record of his cruelties.