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.
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.