If good exists and evil doesn't (metaphysically speaking), why does the will know abhorrence and revulsion as well as delight and attraction? One would suppose that things deprived of goods would simply leave the will indifferent.

Why would one suppose that?

Because evil does not exist in its own right. How can something unreal have such a strong effect? If only goodness exists, though in different gradations, it would seem to correspond more closely if the will only experienced attraction, though in different gradations.