Wesley Blog writes about the chariness some have of talking about evil, especially in terms of the demonic "Maybe we just can't shake the false notion that every being has some good in them no matter how bad they are." I'm going to agree with Augustine here and suggest that such a notion is an important one to hold as Christians.
Augustine's argument in the Confessions is that God created everything good. It doesn't mean that evil is not real. But rather evil is not a thing, not something which has an existence since existence is good and is only possible because of the work of God. Evil instead is a deprivation, something which corrupts that which has some good.
In this sense it's parasitic. You can't have a rotten apple without the good of an apple to work with. And you couldn't have Darth Vader without the jedi knight in him. In the Return of the Jedi you get something few action movies will ever give you: a bad guy who is not pure evil, who can and in the end is redeemed. Isn't that the message and hope of Christian faith?
Is this not taking evil seriously enough? Perhaps, though I don't think externalizing evil into a satan figure gives us a better route. Pure evil calls for no response from us, besides vanquishment, like the old western's theme or today's US foreign policy. But by externalizing evil, the idea of the pervasivness of evil, ie original sin seems lost.
That is, the idea of evil affecting all of life, including ourselves becomes harder to recognize. Instead evil is "out there". It's also easier to imagine folks as all good (ourselves) and all bad (those other folks). It's an old temptation to hold such a view but I think Augustine was right in trying to stear us away from it.