[1] Born and raised in North St. Louis to Irish-American parents, Costello joined Egan's Rats while in his late teens.
Despite the near fatal bullet wound, Costello still managed to stagger three blocks to the east to Willie Egan's saloon and call for a taxicab to take him to the hospital.
Walter also served as a St. Louis City Deputy Constable to Judge Andrew Gazzolo, thereby giving him permission to carry his pistols and even make arrests if he so desired.
Costello was one of two men who shot and killed dangerous former Egan hoodlum Harry "Cherries" Dunn at the Typo Press Club at 712 Pine Street on September 19, 1916.
A few weeks after Cherries' murder, on October 4, 1916, Costello would lead the same hit team into Bev Brown's saloon at 1233 Chestnut Street, where Dunn's friend Eddie Schoenborn worked as a bartender.
Pudgy Dunn eventually succeeded in murdering one of Harry's killers, Frank "Gutter" Newman, on June 8, 1917, in front of a house at 3323 Lucas Avenue.
Walter Costello's killing was ruled justifiable homicide and Willie Egan commented, "I've saved a policeman's life tonight."