Why teaching employee is a very bad idea?

Other competitors promote different things. Employee promote the exact same thing.

Other advertisers research places to make money. Your employee copy you wherever you are.

Other competitors steal market from many advertisers. Employee steal only from you. He can’t copy others.

Other competitor would move on to other places when profit margin is lower than his other biz. Your ex employee knowing nothing else will destroy your whole profit margin.

You can negotiate with your other competitor and both will seek profit. However, you hate your ex employee and negotiating with him will be too unjust.