He was one of very few sadomasochistic serial killers and enjoyed receiving pain as much as inflicting it. He was also a religious zealot.
He attacked and raped numerous children and bragged that he'd "had children in every state". Most of his victims were handicapped or black children, easy targets.
He was convicted of torturing and murdering four-year-old Billy Gaffney and ten-year-old Grace Budd, both of whom he cannibalized. He was connected to three other murders and hundreds of rapes.
His downfall began when he sent a letter to Grace Budd's grieving parents, bragging about what he did to Gracie.
In short, a legitimate reason for children not to talk to strangers.
Another definition is a sick molesting killer fuck who likes children for sex and eating them.
Def 2: Mark is a total Albert Fish