From Stanley Hauerwas' theological commentary on Matthew:

The devil, a fallen angel, is the embodiment of the mystery of disobedience. God would have us love him with the same love that gave birth to our existence. God's love risks our disobedience in the hope that we will freely return the love he has for us. God refuses to coerce us to participate in the love that is the interdependent life of the Trinity. Yet we mysteriously refuse God's peaceable love, preferring to secure our lives by our devices, which inexorably lead to violence against ourselves and one another. Our sin drives us mad because our very ability to revolt against our creator is dependent on the gifts we have been given by him.
That is why the devil is at once crafty but self-destructively mad, for the devil cannot help but be angry, recognizing as he must that he does not exist. Augustine gave classical theological expression to this understanding of sin and evil when he observed that there can be no evil where there is no good. This leads him to the surprising conclusion: "If every being, insofar as it is a being, is good then when we assert that a defective thing is bad, it would seem that we are saying that evil is in fact good, for any defect depends on the goodness that is always prior and therefore there is no evil apart from that which is good. In other words, nothing evil exists in itself, but only as an evil aspect of some actual entity because every actual entity is good [omnia natura bonus est]. Absurd as this sounds, the logical connections of the argument nevertheless compel us to its inevitability."
It is significant, therefore, to recognize that the devil's only viable mode of operation is to "tempt". The devil can only be a parasite, which means that the devil is only as strong as the one he tempts. This is not to suggest, however, that the temptation of the devil is any less destructive for us But it does mean that the temptation Jesus endures is unlike the temptation we endure, for the devil knows this is the very Son of God...

Fascinating insight.