mal-

Proto-Indo-European root meaning "false, bad, wrong." The exact sense of the root remains uncertain, "since it concerns a collection of largely isolated words in different IE branches" [de Vaan].

It forms all or part of: blame; blaspheme; blasphemous; blasphemy; ‌‌dismal; mal-; malady; malaise; malaria; malediction; malefactor; malefic; malevolence; malevolent; malice; malicious; malign; malison; malversation; mauvais.

It is the hypothetical source of/evidence for its existence is provided by: Avestan mairiia‑, "treacherous;" Greek meleos "idle; unhappy;" Latin male (adv.) "badly," malus (adj.) "bad, evil;" Old Irish mell "destruction;" Armenian mel "sin;" Lithuanian melas "lie," Latvian malds "mistake," possbily also Greek blasphemein "to slander." 

"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.