But what gets to me is who could do something like this to someone who could be our daughters or grand-daughters or nieces? I can't imagine that in my mind.
If you really can't imagine it, you're lucky.
If you want to understand sexually motivated murder (which I believe the Delphi ones are), my advice is to read the book Sexual Murders: Comparative Analysis and New Perspectives (eds Proulx et al). Sorry, there's no online link that I know of (I also haven't looked), but this book will tell you what you need to know.
The FBI has outlined the process by which many (not all...but many) murderers develop. In a nutshell, sexual, physical, or psychological victimization experiences result can result in social isolation and favor the emergence of violent fantasies which compensate for the lack of real life control.
Fantasies of coercion and violence become stimuli to these individuals, ones that trigger a sexual response. Feelings of rage and the need to dominate are completely entwined with the sexual response.
If an offender seeks power, when he acts out sexually, this aggression allows him to dominate his victim to the highest degree. Usually because of his own negative experiences, the suffering of others sexually excites this type of offender. And this is also why it's important to understand that this offender is often opportunistically looking for ANY type of victim as a means of asserting his control. He will therefore choose anyone who he judges to be weaker than himself in the moment. (With this knowledge, think back to Carter in the press conference emphasizing "what will your family think....two LITTLE girls.")
This type of sexually motivated offender does not actually need to complete the sexual assault part of the crime to derive gratification. The act of murder itself, when the offender completely controls the life or death of the victim, can be the climax. For days, weeks, months or years after the murder, fantasies and memories of the event can act as a surrogate means of satisfaction.
If the Delphi murders were sexually motivated, this is the type of person who committed them. I could say a lot more on the subject, but that's an overview.