All of the evil guys are Creators too, but in order to do so they need to wipe the slate clean. It's as if you want to build a city that has everything you want in a city, but the land you want to use is occupied. You must destroy their city first so that you can start your city.

Melkor wanted this even in the beginning of time. He tried to weave his own designs into the music of Illuvatar, but he never understood or could comprehend the power of Eru in its totality, so it was fruitless. Eru himself said that even if Melkor thought he was gaining some advantage, he would find himself honouring Eru through his actions. Melkor, did however, have the power to challenge the Valar. When they built mountains, he broke them down etc. I don't agree that he was like a child who wanted to destroy his room, or irritate his parents. Melkor wanted Creation to be as he saw it, and he hated the ideas that the Valar put forward. They felt the same. Melkor diminished himself greatly by being a Creator, which is ultimately how he was nearly undermined on several occasions. He poured most of his power into creating things like desolate lands, and orcs, and other monstrosities, which were ultimately his desires for creation.

The Dark One is the same. He wants to utterly destroy everything, so that he can undo time. For him, the Wheel is an eyesore. He wants to create the anti-Creation, but he can't do it while he's in prison and while the Pattern is sitting where he wants to build his house. Naturally everyone thinks he's the most evil thing ever, especially because he wants to eradicate everyone and everything. However, the Creator would do the same if the DO ever imprisoned him, and he managed to get out.

For me, the highest levels of evil have plausible agendas. They want utter destruction so that they can remake the world into an anti-creation. 'Hell' for us, 'Heaven' for them. Lesser enemies focus on dominion i.e. Sauron, and all the Forsaken except Ishamael. They really are children - they sit around twiddling their thumbs once they've won and don't know what to do next. The greatest levels of good and evil employ destruction so that they can create.