[Ilúvatar: ]Through [Melko] has pain and misery been made in the clash of overwhelming musics; and with confusion of sound have cruelty, and ravening, and darkness, loathly mire, and all putrescence of thought or thing, foul mists and violent flame, cold without mercy, been born, and death without hope.
HoME I 2
From here, we see two types of evil:
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Overwhelming musicsThis, perhaps, is the obvious kind of evil, one in visible form, and the one that causes the most harm.
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Confusion of soundsThis, then, is the subtle kind of evil: "internal" evil, if you will.
The thing is, from this "confusion of sounds" come "overwhelming musics": Melkor, succumbing to his thoughts in the dark, becomes the Dark Lord of the First Age, as fallen as any being could be. Ar-Pharazôn, listening to Sauron's whisperings, brought about ruin in Númenor and the world. Saruman, who wished to wield the Ring for the order of the world, destroyed all that he touched in the end.
In these cases , the first step towards the dark brought about their fall. And they most of them didn't know that they had, in fact, crossed the line, until it was too late to turn back.
Boromir, recognising that he had already taken that first step, repented. But he had to die, because once having attuned to this confusion of sounds, it will continue to play in him until it caused his fall.