Harvey committed the rest of the Holiday killings to cover for Gilda.
Harvey figured Gilda was the one who murdered the first couple of victims and murdered the rest to throw off suspicion, eventually trying to take the blame himself when he became Two-Face. He may have also snapped when he figured out it was Gilda, so zealous he was about finding the Holiday killings.
- This doesn't wash. Note Two-Face's stunned reaction when in the final issue, Batman pleads with Harvey to give himself by invoking Gilda: Two-Face clearly has an epiphany, as he suddenly realized that his loving little wife is a serial killer.
Alberto Falcone got the idea for the Holiday killings from Calendar Man.
Calendar Man somehow contacted Alberto and gave him the idea, so Alberto would become somebody and Calendar Man would be remembered. It would further the Hannibal Lecter parallels, since Hannibal Lecter contacted Francis Dolarhyde to help him.
- No go: As we will later see in Dark Victory, The Calendar Man, incensed that Holiday's notoriety so eclipses his own that The Calendar Man is indeed in danger of being forgotten, orchestrates events to drive Alberto to kill himself.