See "Before Stonewall" for some more heinous details.
Believed in his own past lives.
Involved in various religions or cults.
Co-founder of the Society for Creative Anachronism (need good source)
1950s: lives in NYC and was involved with fandom there.
1953 to 1956: Columbia Medical School "superkids" paranormal research, which included immersing kids in NYC sf fandom; working with William Sheldon. Robert Bashlow may also have been involved. (Hard to source this so far; Jack Sarfatti's forum posts, are, well, unreliable).
1954: convicted of child molestation or lewd behavior in Atlantic City; exposing himself to young boys under a boardwalk. Probation.
1962: Seacon scandal in fandom. Anti-Breen sentiment begins to appear in fanzines (sourcing this should be pretty easy)
1964: marries Marion Zimmer Bradley in June. Publishes "Greek Love" with Warren Johansson under pseudonym. Banned from Worldcon (Pacificon II). Arguments in FAPA over his application on the wait list.
1965: founds the International Journal of Greek Love. MZB publishes an article in it.
1968: Breen and MZB move to Staten Island. They help form the Eastern Kingdom of the Society for Creative Anachronism.
Mid-60s: UCB masters in sociology of music.
1979: Keynote speaker, as J.Z. Eglinton at NAMBLA's second converence in New York. (Notes on the talk in NAMBLA News 2 (June 1979).
In 1980, both Breen and Bradley were ordained in the Holy Apostolic-Catholic Church of the East (Chaldean-Syrian) priesthood by Metropolitan-Archbishop Mikhail Itkin.
1985: visit from K----, the 9 year old son of acquaintances from the SCA. His parents check with Elisabeth Waters, Tracy Blackstone, Marion Zimmer Bradley, Diana Paxson & Don Studebaker about Walter; they all agree the visit is fine. source: http://www.sff.net/people/stephen.goldin/mzb/timeline.html
In 1989 Breen was arrested on child molestation charges for molesting K----, after his daughter Moira tells her psychiatrist of the abuse and K---- confirms it to police. He accepted a plea bargain for pleading guilty to felony charge of activity with a boy 14 or younger, with a sentence of 3 years' probation.
In 1991 he was charged with eight felony counts of child molestation involving a thirteen-year-old boy that he had been abusing for four years. He was convicted, and died in prison while serving his thirteen year sentence.