Oh yea, this fascinates me as well. Mainly in my current life.
People lie. Why do you think I am a cynic?

Knew plenty of liars, cons and cheaters. Still can spot them from 50 paces. I don't know, maybe it is an intuition or an ability to read body language? I do know some of the basic body language things that you can tell if someone is lying.
Are you naive, gullible or "forgiving" aka live in denial? Some people are. They are willingly to "believe" the person because not believing is too painful.
A big one which my sister and BIL is dealing with is denial of mental illness. I find that to be a common thread. Even though it is CLEAR that a family member is mentally ill a person chooses to believe that they are "stubborn". They do not want to accept mental illness as a diagnosis for many reasons.
Those factors play into the aspects of people that are able to con others.
As far as Casey Anthony, the grandma failed her. The grandma knew her dd was messed up. She should have fought her dd to the death to protect that girl. Now she never thought her dd would go that far and kill the girl however at the same time she did not keep an eye on the girl. The grandma was watching this girl since she was a baby and that is why I say she failed her. In her heart she ignored the right thing to do.