The Collaborative International Dictionary of English v.0.48:

Mal- \Mal-\ (m[a^]l-).
   A prefix in composition denoting ill, or evil, F. male, adv.,
   fr. malus, bad, ill. In some words it has the form male-,
   as in malediction, malevolent. See Malice.
   [1913 Webster]

   Note: The form male- is chiefly used in cases where the e,
         either alone or with other letters, is pronounced as a
         separate syllable, as in malediction, malefactor,
         maleficent, etc. Where this is not the case, as in
         malfeasance or male-feasance, malformation or
         male-formation, etc., as also where the word to which
         it is prefixed commences with a vowel, as in
         maladministration, etc., the form mal is to be
         preferred, and is the one commonly employed.
         [1913 Webster]