He had never expected something - good or evil - to actually come out of his wanderings. Deep down inside, he had always known the Flame was only ever with Eru, and yet he searched all the same, never even daring to hope that he might find something. He had never seen the Fire, so for all he knew this was it. Or it wasn't and some power other than the Allfather was attempting to lure him in. Eru would never leave the Flame unguarded; it was too precious and rare to be left alone - especially here; so deep in the Void. But Melkor's lust overcame his weak-opposition. The desire to create was overwhelming in him, and he couldn't just walk away from what could have been, might have been the Everlasting Flame - the power to give souls. He could give life on his own, but he needed the flame to give his creations a soul and he couldn't very well ask Eru for some. The Father had forbidden them from creation; saying he was to create and they were to improve. But Melkor had been created rebellious, and he disobeyed Eru without a thought for that explicit command. Since the Allfather never forbade him from wandering in the Void, he assumed Eru knew nothing of his desire. He was wrong.
Eru knew, and he also knew what lay in wait for the would-be-Vala should his wanderings continue. Yet he did nothing. Arda would need evil in it for the world to be complete and while he knew that the malevolent force that lay in wait for Melkor could not enter out of the Everlasting Darkness of the Void, Melkor himself could and if the evil should possess him, Eä would have the enemy it needed. Eru knew all of this all along, but he told no one and let them believe that their greatest enemy could be cast down by him, for if he were to tell them of their true enemy then he would also have to confess that all he could do against that villain was keep him in the Void and that power constantly hung by a thread, ever weak and always right on the line. When Melkor was cast down after the War of Wrath, Eru felt his enemy leave his creation's soul; giving up on entering Arda through one of his Ainur, he said nothing. Let them drag the lost, mostly insane, and utterly confused Melkor back to Valinor and deal out his punishment; he too broken after so many ages under the complete control of a being far more powerful than Eru himself to even mutter one word of defense on his behalf. Eru stood by and watched them throw Melkor into the Void; right in the hands of his tormenter and did nothing. It was all for the greater good in the end.
Don't ask me where any of this came from because I honestly do not know. I am planning on continuing this, but I probably won't. Ha.