Yes. It's exceptionally unethical in *most* instances unless it's agreed upon prior to the work being performed.

In all probability the *only* reason this "program" was creates was due to the client *paying* you to create it. Any client *paying you* for you to then release the product for free is going to be very, very, very upset.

In addition, the contract for the work may mean you don't own the IP and would be essentially distributing it against the rights you have. No way of knowing this without seeing the contract.

In the end, the *one* thing you can be **absolutely certain** of is such an action will spark horrible word-of-mouth for your services from the client who paid you.