I was recently terminated for processing a transaction that was in violation of company policy. This practice has gone on at this place of business for some time and is easy to prove that no one else has been released for doing the exact same thing that I did. Does this qualify as a discriminatory reason for termination?

Does this qualify as a discriminatory reason for termination? Only if the reason you were terminated and others were not is because of your age, race, gender, national origin, handicap, or some other protected characteristic. An employer simply unevenly applying company policy does not constitute prohibited discrimination.

what about any civil liability? would this be considered defamation or slander?

Not based on the information in your post, no.