Melkor didn't try to disrupt the music just once though. Once could be seen as an honest mistake. But he did it three times and each time he was basically told to stop and yet he kept doing it, griefing the others trying to take part to the point some just game up. Three times is enough to show that he was meaning to mess it up for everyone and try and take over.

As to Eru designing it so that Melkor did rebel, I don't quite see it that way. As someone has GMed a lot, players are always doing crazy things and messing up the plot. A good GM can adapt to that and work it into the story, improving it. That is what I see Eru doing. Being an omnipotent all powerful creator helps, but he also jsut took what Melkor was doing and worked it into the plot as well.