Ilúvatar and Melkor

[Kuruharan]

Far be it from me to sound like I'm contradicting what I said earlier, but are you serious?

Instead he wanted to create and make by his own design and not Eru´s design. Of course he couldn´t (only Eru could create - Flame Imperishable and all) and in his jealousy he destroys much of the works of the Valar. Is that evil?

Well, going around destroying the things of others just because they were not made by you,and causing general mayhem and suffering sounds pretty bad to me.
Since only Eru can create, Melkor was refusing to acknowledge that he was also a created being and that there was somebody who was superior to him.

Is it not merely a trickssy trick of Eru, giving Melkor the greater power of the Valar and seeing him bend and crack under the weight of his own power.
So Melkor was not evil as charged, but only acting according to his nature given to him by Eru.

Now we get back to the whole "fated to fall" question. I personally do not believe that things are destined to happen a certain way and that there is free will. So, Melkor had the choice to do the right thing or do the wrong thing. He did the wrong thing but this was allowed to stand, because to do otherwise would interfere with Melkor's free will, and in the End greater glory could be achieved.

assuming Eru was mightiest of all.

Yes, let's assume this.

So Manwë was given understanding but not power; he knows but can´t act

Or he acts within his limits, insofar as he understands them.

Melkor was given power; he knows not but acts.

And he only acts on his impulses of self-aggrandizement and personal power to the ruin and misery of the world.