I was just looking this up! A little bird told me you’d ask this question. I said I thought that one thing they had in common was some sort of severe abuse as a child, and that seems to be the case. From this Wiki article, “They were often abused—emotionally, physically, or sexually—by a family member.” Ted Bundy has some attachment disorder, “possibly due to his mother passing herself off as his sister during his childhood.” I read “The Stranger Beside Me by Ann Rule. She actually worked with Ted Bundy. If I remember correctly, his grandfater was his actual father, so in a way his mother was his sister as well as his mother. But I think it was just hinted at as he was growing up.