[Eldar14]

When Ilu created the men, he is depicted as sad, as if he doesn't really want to bring men upon his great creation. When Melkor first started to twist the music, botht the elves and men were attempts to stop him, and I think that elves were the kind of pacifist attempt at stopping Melkor, destroy his evil by making the beauty and greatness of the music stronger. But when that failed, he created the men, the offensive approach, with their baying of trumpets would destroy much of the beauty, and even be turned to evil themselves (which may have been part of the plan), but be so offensive that they would overwhelm the dissonance and evil. Also, Ilu told Melkor that all of his attempts to destroy the music ended up going towards the greater beauty (such as snow), so maybe he created men with the intention of them being turned to evil, just to show Melkor that no matter what he corrupts and twists, it won't destroy the beauty of the music.