What is the ruling on defaming a bad colleague from a previous job with the intention of warning people about him?

The basic principle is the prohibition of backbiting and mentioning faults. Allah, the Almighty, says: "And do not backbite each other." It is not permissible to mention flaws unless there is a legitimate and recognized Shar'i interest. So, if someone knows of a flaw in their colleague, they should not expose them unless someone intends to deal with them. In such a case, the flaw should be mentioned to the extent necessary for warning and advising. Similarly, it is obligatory to disclose a flaw when consulted about marriage or transactions, while being careful to maintain sincerity of intention and good purpose, and to beware of being motivated by malice or envy.