Tu as raison -- mal est un adverbe, l'opposé de bien. Mauvais est un adjectif, l'opposé de bon.

Generally mal is used as an adverb, but it can also be an invariable adjectif (yes it is confusing...) -> Il est (au plus) mal = can mean he is sick or dying or in a dangerous situation or in dire straits etc... It is also used instead of mauvais in the expressions : bon an mal an / bon gré mal gré It can also be a noun : le bien et le mal (good and evil, in this case mal=evil).