Since WizSec's last report was published, the theory Mt. Gox's bitcoins were lost over a period of years beginning as early has 2011 has gained some traction. If so, it suggests your insider must have been active over a long period of time. From what I understand, there were only ever 30 people including contractors working there. If the insider was not Mark, then s/he should be easy to indentify. Who was this sneak?
My thoughts would go to an unhappy employee. People can do various things when they're unhappy and at some point the business in MtGox was booming. Too many transactions, too many responsibilities, probably the worst part was an autistic MK towards his anxious people. Frankly, I cannot say for sure. It's only a hypothesis that is yet to be proved right or wrong.