I find it strange that the second victim had her face blown off with a shotgun, whereas the other three were all strangled/suffocated to death. That's quite a radical shift in MO. In some twisted way the killer cared for his victims before he disposed of them. Strangulation makes sense, because it's an 'intimate' way of killing someone, it's up close and personal, or least it is compared to shooting someone in the face. What would possess him to suddenly change like that between murders?