There must be a contrast because without such, every choice is indifferent, because there will not be a good or bad choice. There will also be no difference between living and death if they are not the opposite of each other. Many modern philosopher had envisioned a place where only happiness exists (a Utopian society), but has ultimately failed because happiness is undefined without its opposite.
For the sake of arrangement, if, say, an ultimately "good" god creates lucifer. At the time of creating him, lucifer was good and did a number of good deeds and was god's favorite. However, many eons later, lucifer turned into the greatest evil in the universe. Can we say that god is evil? I don't think so, because at the time of creation, lucifer was good. It's much like the mothers who gave birth to Hitler or Stalin -- are they inherently evil?
Also, If god is omniscient, he knows that lucifer is going to turn. However, if creating lucifer is a necessary condition to achieve a greater good, is that still evil? We as mortal beings cannot see a billion years into the future, so obviously we cannot envision what that future will be like.