A thing acts in virtue of its nature, and it acts for the sake of its own perfection. Perfection, then, is what is desirable, and something is only desired inasmuch as it grants some end or perfection. However, some ends can only be had to the exclusion of others, meaning that one form entails the privation of another form. This is how a creature may desire evil, not in itself but inasmuch as it is related to some good. In other words, evil is only a principle of action and desire per accidens. Only good can be a principle per se.
Evil cannot be a first principle.