Hilchos Lashon Hara chapter 10, section 11

Be very careful not to tell others: "I did business with so-and-so, and he cheated me, robbed me, shamed me or hurt me," and the like. This is forbidden even if you know the story is true, and even if the leniencies above apply. (The exception is the case explained below in paragraph 13.)

Why? In such a case your goal is surely not a helpful one, such as getting people to reject evil, or getting the person to change his ways. Instead, you want to put him down because of what he did to you. And the more people shame him, the happier you are.