[Mandos]

Perhaps Eru did not MAKE melkor evil, but rather LET him be evil.( an important thing to reme,ber here is that Eru gave to the Ainur independent wills and thoughts, for he wanted to see his creation grow) He knew what all would happen , but yet he allowed it. Like mortals, we are not meant to know why these things happen. For why did God cast Satan down to Earth rather than destroy him?

It is illogical to assume that good would exist without evil. For can light exist without dark? For everything there is a postive and a negative, that is physics and not debatable.

Eru knew that Melkor would wreck havoc upon Middle-Earth and its inhabitants, but he allowed to it. Why you may ask? You might as well ask why do our loved ones die, or why there are mass murders and world wars where women, childeren, and the elderly are brutally slaughtered. Possibly, to teach us all a lesson which may save us all one day, or to give us an understanding into something that may be worth it for the greater good.

In any case, there is always something bigger and better than us in charge of affairs that we as mortals in our short lives and two-dimensional vision do not comprehend and very likely can't comprehend.

Light and Darkness, Death and Life, each is necessary for the other to exist.I am just thankful that someone who knows much more than me and is greater than us all is in charge. For who are we to question the ways of things that we do not even understand? Light and Darkness, Death and Life, Good and Evil; each must be in balance and each must exist.
Eru let Melkor be evil so that Middle-Earth would become what it was. Without his influence things would be much different. Better you might say? Perhaps, but we do not know that. Perhaps without the influence of this Vala, a greater evil would have happened?