Let's say you are an employer and there is a clause in every worker's contract that they should have a good moral character to remain in your company.
It depends on what you mean by moral character. It is subjective if also include gambling to fall under moral character as owner of gambling company. Some people don't consider gambling as immoral.
Then you found out that one of your best employees who happens to be your best-performing employee is addicted to gambling and spends a lot of money on casinos.
If the clause includes gambling prohibition for my staff for example and like you said, a best worker is found to be violating the company's rule severally, of course he or she has to be fired. If he is left there, he will become a bad egg and corrupt other workers. Despite being the best worker which he could be using as advantage on the employer, he has to be fired.
What employers do with their best staff is to attach another worker to the best person so that he can also get same knowledge from the best hand and afterwards they are fired.
Moreover, such best work might be as a result of his work experience not only qualification, so another worker who is exposed to such number of experience can also acquire the knowledge to assume another position of best worker. So a worker who refuse to stop violating rules of a company has to be fired, nobody is indispensable
Will you fire him because you think that those addicted to gambling do not have a good moral character and should not stay in your company?
Not because those addicted to gambling has no good moral character but because he may be a bad influence on others. In an off line gambling, usually the workers are encumbered to gamble and if they are caught in the act irresponsibly, they are fired or their phones and properties are seized. This reason for this is to ensure that they don't gamble with company's money.
Or you will just ignore it because you cannot afford to lose a good employee, because you worry that sales might suffer if you cut him from your company?
Nobody is indispensable. There will be other good hands if exposed to same training that the " best"" worker has gone through.