more then likely, he will try those active accounts if he finished those inactive ones.
remember he's into gambling. he wants more money...
the reason why i transferred my btc to my wallet, even if i really trusted my exchange
youll never know whats going to happen
Truth. And i would expand it and say that not only exchanges but any web/browser wallet for the exact same reason.
If you don't hold the keys yourself there is a fair possibility that you will have troubles in the future. Some way or another. In this case, sadly it was an internal enemy, not that easy to see coming. Reason why internal security and limited permissions for employees are key in industries.