The Holiday Killer's modus operandi was killing gangsters, criminals, and corrupt officials all connected to Carmine Falcone somehow, making Holiday a vigilante of sorts.
Gilda Dent (ex-wife of Harvey Dent, the future Two-Face) has also claimed to be the Holiday Killer, and also indicates that Two-Face replaced her as the killer; though Alberto later admits to committing all of the Holiday murders, including that of his cousin, Johnny Viti. His father offers to use his influence to free Alberto, if he only admits to killing Sal Maroni, a fellow mob boss and rival to Falcone. Alberto refuses, however, smugly saying that his new reputation as Holiday makes him much bigger than his father and all the gangsters in the mob put together. He is convicted by the jury and sentenced to the gas chamber. Although Alberto initially receives the death sentence, his unstable mental state allows him to instead plead insanity. He is imprisoned in Arkham Asylum instead, across the hall from the Calendar Man, another holiday-obsessed villain who is afraid that Holiday might overshadow him.
Despite the warnings of Commissioner Gordon, Alberto is released from Arkham with the help of Janice Porter, Dent's replacement as Gotham's district attorney. He is placed under house arrest at the Falcone compound outside Gotham, with a tracer is attached to his leg to prevent escape. While in the house, Alberto is manipulated by The Scarecrow and Calendar Man into believing that he is being contacted by Carmine's ghost; the Scarecrow, a former psychologist, determines that Alberto is most afraid of his father, and the Calendar Man poses as the Roman's 'ghost' to permanently eliminate his 'rival'. The gun from the Holiday murders is soon left in his possession, which he uses to save Sofia from The Joker. On Carmine Falcone's birthday, Alberto receives another gun to kill Sofia in order to usurp powers of the five crime families (actually a part of Two-Face's plan to destroy the crime families). Alberto nearly kills her in a Holiday-style shooting, but ultimately stops himself. Following Janice Porter's death, Two-Face has her body left in Alberto's bed in order to trick him into thinking he had killed her. This time Scarecrow uses his fear toxins to make Alberto hallucinate that his father's voice is commanding him to commit suicide. Alberto then knows that the voice is not real — his father did not believe in suicide — and he exposes Calendar Man. What happens next is never explicitly shown, but later events suggest that Calendar Man shoots and wounds Alberto, at which point Sofia Falcone attacks Calendar Man, breaking his jaw and attaching her brother's electronic monitoring ankle collar to him, leaving him unconscious outside the Falcone estate for Batman and the authorities to find. (Gordon and Batman appear doubtful that Alberto could have injured the Calendar Man so badly himself.) Sofia then takes her wounded brother to a hiding place in the Falcone mausoleum. There Sofia criticizes Alberto for complaining about the pain, recalling how Carmine had survived five shots to the chest. Alberto replies that he is not his father. Sofia, disgusted, agrees with this assessment and smothers him to death.
He will later appear in the game Batman Arkham Origins, a prequel to Arkham City.