There was a story a while back about a law firm that discovered its escrow account was empty, even though it should have had millions in it. Turns out a legal secretary had gotten caught up in a Nigerian scam. She started out sending a small amount of her own money. They'd tell her they just needed a little more and then she'd get it all back plus a lot more. As she got deeper in trouble, at some point she started "borrowing" from her employer. Incompetence led to pressure that created ethical failure.
My guess (as uninformed as anybody's) is that Karpeles started out merely incompetent and with underdeveloped ethics. As he got in deeper, he just focused on trying to stay out of trouble in the short term. That led to massive snowballing of problems. His very modest helpings of competence and ethical backbone were quickly overrun, demoted in priority to the same level as getting enough niacin in his diet. The best liars I know aren't people who know that they're lying; they're people who focus on saying what you want to hear while ignoring entirely what the truth might be.
For those unfamiliar with the Dunning-Kruger effect, it also seems relevant here: http://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect