According to St. Augustine, evil is the absence of good. He says that evil is not a thing in itself, but rather a negative -- just the absence of good.
In Augustine's dialogue with Evodius, he introduces us to the concept of inordinate desires. To him, these desires are the cause of all wrong-doing. What makes a desire ordinate or inordinate is the object of that desire.
The object of an ordinate desire is something that can not be taken from you without your consent. The object of inordinate desires are things that can be taken from you without your consent.