[Legate of Amon Lanc]

Originally Posted by Lalwendë

This world is not corrupted by evil, it is created from the very beginning with evil inbuilt, into the very fabric of its being. There is no 'paradise' from which the Children can fall because the world was fallen before it even began.

Actually, I dare say not. If by the term "world" you mean just "Arda", then you are right: the evil was inbuilt in it. But in our case I'd change the term "world" for "creation" here: because this is what we are dealing with if we ask for Eru. Arda was actually not created just by Eru, it also (and as we know, VERY MUCH) contained thoughts made by Manwë, Varda, Melkor etc.: and this happened at the point in which they were individual entities. They were no way linked to Eru at that time: it was their own ideas what they put into the world. Melkor brought in the dischord, and this way, evil was inbuilt to the history of the newly created World. We all know the story of Ainulindalë, and what Eru said to Melkor. When Eru put the plans of the Great Music before Valar, there was nothing of what Melkor later did. In the beginning, there was nothing evil: not even Sauron, as it was said here many times. Eru created beings (meaning the Ainur now) with completely free will. He gave them their powers, yes, but he did not imprint in them "You Melkor are going to behave like this and this". And since there was no dischord in Eru's original plans for the Music of Ainur, we have to conclude that everyone of the Ainur behaved as he wished: and Melkor has chosen to behave differently.

Then Ilúvatar said to them: 'Of the theme that I have declared to you, I will now that ye make in harmony together a Great Music. And since I have kindled you with the Flame Imperishable, ye shall show forth your powers in adorning this theme, each with his own thoughts and devices, if he will. But I win sit and hearken, and be glad that through you great beauty has been wakened into song.'

ye make in harmony together=in harmony, no dichord was in the plan
I have kindled you with the Flame Imperishable=they are now "real" beings with their own life (cf. Dwarves when Aulë tries to get them working without Ilúvatar)
each with his own thoughts and devices, if he will=their own thoughts and devices, since they are free beings. Ilúvatar gave them powers, let's say, "you have now the ability to create nice bricks, you have now the ability to be a good architect, you have the ability to be a good gardener, etc., etc., now make me a nice house with garden". You can build a house in Victorian style, in postmodern style, as you wish - it's up to you if you put one or two doors there. But if you decide to build a rocket silo instead, it was not what I asked of you. (I am sorry, I am neither an architect nor a mason, but I hope you catch my meaning.)
and be glad that through you great beauty has been wakened into song.=I don't consider dying children, or dying Thingol, or the Kinslaying, or Saruman's destruction of the Shire, or death of Denethor being beauty. This was not Eru's plan. But when it happened, he accepted Melkor's free will, as much as he accepted the free will of anyone else: this is justice. And what he said to Melkor, we know.

Then Ilúvatar spoke, and he said: 'Mighty are the Ainur, and mightiest among them is Melkor; but that he may know, and all the Ainur, that I am Ilúvatar, those things that ye have sung, I will show them forth, that ye may see what ye have done. And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined.'

I know this is the source of "Eru wanted Melkor to do this!" opinions, but I say, nay, as I said above, Eru didn't want this in the first place, and if you read this quote carefully, you'll probably see for yourselves. Eru is not saying "You cannot play anything which I didn't want to do". Eru is saying: "But that ye may know that I AM ILÚVATAR" = so there is no way you could make anything which will overthrow me, most crudely said. It does not mean "if you play this, Melkor, it means that I wanted to play this as well" but "you cannot play this. And you cannot play anything new. I created all. You might choose whether the grass would be green or yellow or pink, but you cannot invent a new color. You might choose whether the balrogs will or will not have wings, but you cannot invent other shape for them than putting the bull's head or pig's head together with human body." I know this is bad example, since the Ainur actually created even the shapes, but since we are bound by the material world and cannot imagine invisible things, I think this is the best.
Someone might point out (and I think you, Lalwendë, inclined to that) that Eru surely is not "stupid" and that he could've known that the evil would come and that he could have, crudely said, for example "erased" Melkor. I think it is like this: if you create something with its own free will, you have to count with that it may - or even be sure that it will - do things you didn't want it to. But is it better just to sit in the void and not create anything? Please leave now aside that the World itself was created after the creation of Ainur: we know that Melkor had the thoughts of dischord in him even before the Music itself. These were his thoughts. But if you look into the world after creation, Eru also wouldn't censore every single baby's genetic structure to make sure that no possible thief is born. They have free will, if they want to steal, it is their choice.
So, if I sum it all up: before the Great Music, Eru didn't know what will the Ainur play. He also didn't know what all would happen: even if he would have some suspicions on Melkor, he couldn't ve known if he would play alone or if some Saurons and Balrogs would join him, or if he would be "overshouted" by the others. During the Music, when it seemed really bad, he intervened (!). And when it was probably unbearable (?), he said "ok, that's enough of suffering - let's cut it" and he ended the Music. And after the Music, he didn't just say: "Okay, I wanted this to be a nice piece of music and I wanted to create a world according to it, but you screwed it up, so on second thought nothing," but he said: "Okay, Melkor, so you screwed it up, but that you all know that I am Ilúvatar, now watch what you have done."