Sadistic personality disorder is a personality disorder involving sadism which appeared in an appendix of the Diagnostic and Statistical Manual of Mental Disorders (DSM-III-R). The later versions of the DSM (DSM-IV, DSM-IV-TR and DSM-5) do not include it.
Why not? They have listed everything in that psychiatric book, so why not sadism? Even a little thing like “not trusting the government” is officially a disorder, but enjoying pain for others is not? Both exist as part of a character. And I consider the later a problem. Distrust towards the government is a sign of a healthy mind, at least often it is (yes, we agree not always).
One might even say: When you can’t spot sadism as a disease as a professional group in power of institutions, you are unwilling to see it and call it a disease, for reasons unknown (but we could guess). This profession decided to not see or not consider (strong) sadism as a disease.