He needs to learn a valuable lesson. don't shit where you eat. The question isn't about firing him. Do you really want someone you can't trust working for you? If he's willing to stab you in the back personally you don't think he'll stab you in the back professionally? Do you really want someone that has no ethics working for you? Do you really want someone that dumb working for you? The only question, how hard/long is it going to be to get others crossed trained on his job until you can fire him?