Question: Which mass murderer was the Wilson brother who sang on "In the Back of My Mind" associated with?

Context articles: Fritz Angerstein
Seth Privacky
Omar Mateen
Dennis Wilson
Lorne J. Acquin
I Can Make You Love Me
Akiyoshi Umekawa
Shell Lake murders
In the Back of My Mind
The Stepfather (1987 film)

Fritz Heinrich Angerstein (January 3, 1891 – November 17, 1925) was a German mass murderer, who killed 8 people at his home in Haiger, on December 1, 1924.
 The subject of a media spectacle, Angerstein, along with Fritz Haarmann and Peter Kürten, is considered one of the "three great mass murderer trials" of the Weimar Republic-era of Germany.
Seth Privacky (June 2, 1980 – July 15, 2010) was an American mass murderer who was convicted of five counts of murder at the age of 18 for the shooting of his parents, brother, brother's girlfriend and his grandfather in Muskegon, Michigan on November 29, 1998.
Omar Mir Seddique (November 16, 1986 - June 12, 2016), also known as Omar Mateen, was an American mass murderer who killed 49 people and wounded 58 others in a mass shooting at the Pulse gay nightclub in Orlando, Florida, on June 12, 2016, before he was killed in a shootout with the local police.
 It was both the deadliest shooting by a single shooter and the deadliest act of violence targeting LGBT people in United States history.
Dennis Carl Wilson (December 4, 1944 – December 28, 1983) was an American musician, singer, and songwriter who co-founded the Beach Boys.
 He is best remembered as their drummer and as the middle brother of bandmates Brian and Carl Wilson.
 Dennis was the only true surfer in the Beach Boys, and his personal life exemplified the "California Myth" that the band's early songs often celebrated.
 He was also known for his brief association with then-aspiring songwriter Charles Manson, who was later convicted of murder conspiracy.
Lorne J. Acquin (born March 21, 1950) is an American mass murderer who killed his foster brother's wife, her seven children, and their niece in their home at Cedar Hill Drive in Prospect, Connecticut on July 22, 1977 by beating them to death with a tire iron.
 He afterward set fire to the building and escaped, but was arrested the following day and sentenced to 105 years in prison two years later.
I Can Make You Love Me, also known as Stalking Laura, is an American made-for-television film starring Richard Thomas and Brooke Shields.
 The film is based on the real-life story of American mass murderer Richard Farley, a former employee of ESL Incorporated whose romantic obsession and subsequent stalking of co-worker Laura Black culminated in the mass murder of several co-workers at ESL's headquarters in California, resulting in the first anti-stalking laws to be enacted in the United States.
 The first half of the film covers the events of the stalking and Black's unrequited love that led to the shootings, while the second half deals with Farley's rampage on the ESL offices.
Akiyoshi Umekawa (梅川 昭美 , Umekawa Akiyoshi , March 1, 1948 – January 28, 1979) was a Japanese mass murderer who killed a woman on December 16, 1963, and shot dead four people on January 26, 1979.
 Mass media also used a number of aliases, including Teruyoshi Umekawa, Terumi Umekawa, Akimi Umekawa and Akemi Umekawa.
 He was one of the rare criminals who was shot dead by Japanese police.
The Shell Lake murders is the name of a single mass murder incident committed by Victor Ernest Hoffman (b. 1946, died May 21, 2004) in Shell Lake, Saskatchewan, Canada, during the early morning of August 15, 1967.
 Nine people, all members of James Peterson's family, were shot in the head by a man who was later called "Canada's worst random mass murderer".
"In the Back of My Mind" is a song written by Brian Wilson and Mike Love for American rock band the Beach Boys, first released on their 1965 album "The Beach Boys Today!
".
 It is a heavily orchestrated ballad in 6/8 time.
 Dennis Wilson largely sings lead unaccompanied though briefly during the middle-eight, his brothers Brian and Carl sing two lines in unison.
The Stepfather is a 1987 American psychological horror thriller film directed by Joseph Ruben and starring Terry O'Quinn, Jill Schoelen and Shelley Hack.
 O'Quinn stars as Henry Morrison, an identity-assuming serial killer who remarries a widow with a teenage daughter.
 After previously killing his family and changing his identity, his killing spree continues after his stepdaughter becomes suspicious about him.
 It is loosely based on the life of mass murderer John List, although the plot is more commonly associated with slasher films of the era than a true story.
 The film was written by Donald E. Westlake, from a story by Westlake, Carolyn Lefcourt and Brian Garfield.