God created everything, and everything God created was good, yes? therefore God must have created magic, which is therefore inherently good.
You might be thinking now though "But if everything God created is good, then where did Satan and his demons come from, and why is there evil in the world?" Well, if you actually take a second to read Genesis you would have the answers to both questions. Satan, ( or Lucifer ) was originally an seraphim, the highest ranking Angel on the Angelic Hierarchy ( his name, Lucifer, is actually Hebrew for "Lightbearer" ) who turned against God at the very moment of his creation, along with three-quarters of the other, lower ranking angels. Why does evil occur in the world then? Well, let me answer your question with another question, does Darkness exist? and if so, what is it?
Darkness, in fact does not exist. Darkness is simply a word invented by humans to describe an absense of light. This is true of evil as well. Evil is just a word invented by humans to describe what happens when men forget to live in the light of Christ and follow his teachings.
As magic is naturally good, the mage who uses magic for evil or corrupt purposes is going against the magics natural predispostion, and as such there magic is always slightly weaker than they wanted. The Mage who uses his/her magic for good, however is going with the predispotion of the magic, and therefore their magic will be stronger than that of an evil or corrupt mage.