Elizabeth's husband had friends with a rather... unusual... career. On the evening of the 21st, her husband found an excuse to take her sister out of the house and leave her alone with John and Robert. The courts had to suppress a lot of testimony pertaining to their background - John's youthful membership in a neo-Nazi sect, Robert's desire to get a swastika tattooed on his forehead.
Elizabeth's husband had, apparently, told her a little too much about their career. The gang's social circle believed that their income came from thefts and benefit fraud. This was true enough, but it wasn't the whole story. John would select the targets, and a group of his followers would kidnap, torture, and murder them.
Ironically enough, Elizabeth was the one who gave them away. Living, she kept her husband's foul secret. It was her disappearance that ultimately led the police to investigate the gang - and find the place where they kept most of the corpses.
If she had reported her husband when he first told her about the gang's activities, she would be alive now - and so would their final victim, a young man the gang murdered and cannibalized. Of course, the same could be said for several of the gang's victims - John would often recruit new members, who would then become security risks and wind up as the gang's next toy.