now one person with no moral values could destroy the economy at any moment.
That's assuming a lot.
For instance, I don't have morals applying to the harm of animals, but when people hurt plants I tell them to stop.
Maybe this person just has no morals applying to the money in exchanges, but would see a moral problem with tanking an economy.
I disagree, assuming that the thief is looking for more money is not "assuming a lot": he could mix those coins, sell a fraction just to be a new fiat millionaire, do shorts then drop, talk with some evil banks that wants to destroy bitcoin, etc.
If MtGox was actually robbed as they claim then now we have one single actor that have the power to destroy the economy anytime.