"wrongful conduct, the doing of that which ought not to be done," especially "official misconduct, violation of a public trust or obligation," 1690s, from French malfaisance "wrongdoing," from malfaisant, from mal- "badly" (see mal-) + faisant, present participle of faire "to do," from Latin facere "to do" (from PIE root *dhe- "to set, put").
[S]pecifically, the doing of an act which is positively unlawful or wrongful, in contradistinction to misfeasance, or the doing of a lawful act in a wrongful manner. The term is often inappropriately used instead of misfeasance. [Century Dictionary]
Malfeasor "wrong-doer" is attested from early 14c. Related: Malfeasant.