Mal- \Mal-\ 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 formmale- is chiefly used in cases where the c, 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 malis to be preferred, and is the one commonly employed.

Source: Webster's Revised Unabridged Dictionary (1913)