I guess one of the problems we have is our use of language and its application to certain philosohical concepts. The limitations of this creates certain conundrums that can be exploited to justify any contravening idea one wishes to express. In this case the idea of God being simultaneously the creator of good and "evil".
God of course, being Ulitimate Holiness, did not create evil, because in a sense "evil" actually cannot be created. It is the natural consequence of the removal of that which is created, good. This is where semantics breaks down and philosophical concepts become muddied.
But lets take an example from physics. It is well known that "cold" is a term we use to describe a condition that exists. But that which exits is not "cold", it is only the word we use to describe this phenomenon. The thing that actually does exist is HEAT. Now, being an energy, we have learned to manipulate heat for our benefit. By reducing heat, we "create" a condition that is a natural consequence of a loss of heat. We use a word to describe this - Cold. But remember we have not created cold, because "cold" is not an independantly created condition, but what we have done is we have actually manipulated a thing that is created, heat. To arrive at a point where heat no longer exists, we need to reduce the temperature to -273.16C [-459.69F] At this point all we have is cold. But "cold" as a phenonenon on its own cannot be created. To arrive at this point, Absolute Zero, there is nothing we can do to cold, as if we can "add" to it. What we have to do is constantly keep removing heat, till no more heat exists.
May I then suggest that "evil" is the same idea. You cannot create "evil" but what you can do, because of the divine gift of free will, is manipulate "good" - the thing which is really created. The more you remove good, the more "evil" is manifested. The absence of "good" is what we call "evil" but remember that "evil" is the semantical way we have to describe a condition for which we are, in reality referring to the absence of good.
"Evil" which is a condition that describes the "removal of good" does exist, just as "cold" does. The Bible injects a moral precept into the lifeblood of the human psyche. Just as "cold" ultimately brings death, so, the Bible warns, the removal of good, brings a worse kind of death.
Cheers