A gamble addict should not be in charge of such position of a teller because she will end up looking for a way in which she will feed her addiction because she lacks self control. Addiction is something very dangerous which controls the gambler mindset and thinking to use whatever money in his possession for gambling despite the consequences.
One thing that people don't know is no matter how long you have been doing something it must reveal someday. The teller thought she was playing a smart one unknown to her that it was a dumb idea because using people's money to gamble shows a sign of irresponsible gambling.
Addicts cannot be trusted with money, and that is because in their state of mind they believe they can get away with anything thinking no one is ever going to find out, and even if they can get away with it for some time, eventually they will get discovered, and since this is a massive breach of trust, it is possible she will find it hard to get another job, as who will trust a person that stole from their clients and the bank in this way?