For a lil nightmare for Christmas, head down to Zak Bagan’s Haunted Museum in Sin City … where he’s just unveiled an exhibit for a Santa costume once worn by an infamous serial killer.

The “Ghost Adventures” star tells TMZ … the costume worn by John Edward Robinson — a convicted serial killer, con man, embezzler, kidnapper, and forger – will be on display through the holidays at his museum.

The Santa outfit was the one worn by Robinson to children’s parties, scouting events and other similar functions. Zak bought the costume a few years ago and came from John’s estate.

But, even Zak’s creeped out by his new possession … displaying his serial killer costume inside his museum’s “Jail Room” — built from actual bars dissembled from an Iowa prison. He says anything belonging to these evil monsters should be behind bars where they belong.

Robinson was indeed pure evil … having been found guilty in 2003 of 3 murders in Kansas City and receiving the death sentence for 2 of them. He also admitted responsibility for 5 additional homicides … with investigators fearing there might be other undiscovered victims.

His crimes were particularly spine-chilling as he made contact with most of his victims through online BDSM-themed chat rooms, … often going by the name “Slavemaster.”

He was given the label of “the Internet’s first serial killer.”