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Like Radaghast said, it was Morgoth trying to take over the song by using his own theme, not the one that Illuvatar gave. Similarly to Adam and Eve when they ate the forbidden fruit out of desire to have power like God, Morgoth wanted to make his own theme and be master of the song, even though that role belonged to Eru. Instead of contributing to the song he sang discord that harmed the other Ainur, and himself as well in the end.
Sorry if I'm just repeating what Radaghast said.