Thomas Aquinas 
On Evil [PDF ebook] 

The De Malo represents some of Aquinas’ most mature thinking on goodness, badness, and human agency. In it he examines the full range of questions associated with evil: its origin, its nature, its relation to good, and its compatibility with the existence of an omnipotent, benevolent God. This edition offers Richard Regan’s new, clear readable English translation, based on the Leonine Commission’s authoritative edition of the Latin text. Brian Davies has provided an extensive introduction and notes. (Please note: this edition does not include the Latin text).