Eru letting Melkor go..

[Salocin]

Melkor's influence was part of the pre-existing landscape of ME, and the Edain had to navigate around it, or founder and be pulled down by it.

Without darkness, could we enjoy, understand, appreciate the light?

Without the storm, would the calm mean as much?

Without evil, would "good" be so, well, good?

Perhaps Eru, in his wisdom, knew that in order for creation to have deeper freedom, there was need for Melkor and the evil that he brought.

These opinions of God/Eru make him up to be a monster. What kind of God would set the world op for suffuring? Only a malitious God which most of us would agree caracterizes neither God nor Eru.

You guys are looking at it the wrong way.
This:

Creation was suddenly given a choice...although it sounds strange...they were not "enslaved" to good. They were able to chose Eru and the way of the Valar, or Darkness and the way of Melkor.

is more on track.

God could create a perfect Utopia were a bunch of mindless zombies always did good. Even a good programmer can do that. Instead God created something much more increadible. He created a world were people had free will. If you really look at it free will is an increadible thing. But in order for people to have free will people can choose to follow the purpose of the creator (good) or not (bad). Neither Melkor nor the Devil "created" evil. Neither did God. It was not preasent before the creation of the world like God was. It is mearly an opposite. In the same way that darkness is an absense of light and cannot be created and did not exist before the creation of light (it could not be defined with out light) so evil is an absence of good, cannot be created and did not exist before there were people to do, or not do, good.

Melkor/the Devil was given free will just like every body else, he was just the first to not do good (ie. do bad) he thought his own purposes were better than God's. Some of what you guys said made it sound like Melkor, at least, was predestined to be evil so as to provide an obsticle for the children of Illuvaltar so as to "weed out" the bad one's.