Serial killer groupies always existed. Except for the Manson Family, we tended not to pay attention to them.
Historically serial killers and mass murderers have always had their groupies. Explaining that phenomenon requires four psychiatrists and a Bell Jar. Human beings have deep wells of depravity and there is a very specific type of mind that is attracted to a murderer. The phenomenon of teenage girls who fall in love with serial killers and liberal societies who fall in love with Islam is not so different.