I think the difference among them as far as Hacking goes is that Mark Hacking actually possessed a conscience. I don't think he was a sociopath. I think he was mentally tortured...felt so inadequate and unworthy as the person he really was that he had to invent something better. I don't think it was because he was narcissistic. I think it was the other extreme. He felt he wasn't good enough. For a lot of reasons, I think there was more to his actions than just wanting to get someone out of the way. I wish we could've seen some psychological evaluations.
The reason I say he had a conscience, unlike the others, is because he did confess and give details and he didn't try to get out of it. He knew he needed and deserved to be punished, and he cared enough about his family and Lori's that he didn't want to add even further to their grief and misery by dragging it out and trying to get off the hook. That's how I see him, anyway, just my own personal feeling.
He's the only one I've ever felt a tiny bit sorry for -- not that he should escape punishment in any way, but I felt sorry for him that he viewed himself as completely worthless and felt he had to invent things like having interviewed at different medical schools and having been accepted at UNC. It came to a head, a point where he couldn't keep fooling everyone, and he snapped like a twig under that realization and literally lost his mind in his panic to try to "fix" things.
I'm NOT defending him whatsoever...I just feel he is not quite in the same class as some of the others. I have no idea what could have possessed Casey to do whatever she's done with Caylee, but her demeanor is much more like Scott Peterson than the others mentioned, IMO -- arrogant, thinking she can fool everyone and involving her family in the whole mess, to boot.
Thing is, I think Peterson is actually much smarter (relatively speaking) than Casey...and look how things turned out for him.
I don't see Casey getting away with anything she's done in the long run. She seems to be a pathological liar, IMO, but a pathetic one who isn't clever enough to sustain her lies and formulate any sort of plausible scenario to make this all go away, as she'd hoped to do.