The rantings of chaos are best ignored. Remember that he that hides a dark soul and foul thoughts walks benighted under the mid-day sun; Himself is his own dungeon. While your words offer freedom, what do you think you are offered in return? At best a dungeon all of your own.
He's not mad, it's just an attempt at how Eldar actually insult people: poetically. Unfortunately...it's poorly founded. Although I appreciate the attempt at a poetic retort, the present members of Chaos are not exactly hiding a dark soul, thus you're internal logic is fallacious. One is not 'benighted' if they aren't hiding their sins, they're just loud sinners, so the rhetorical question of their 'dungeon' is moot. They often try to turn the Eldar to Chaos, and this is where your metaphor is relevant: in warning other Eldar away from corrupting themselves and dealing with the guilt of effectively turning on their kin. In arguing that the minds of Chaos worshippers are 'dungeons', it is inappropriate to assume they must 'hide' their dark nature.
you cannot even remember your own threats, they are as empty as your body is of a soul. perhaps you did not know what you threatened showing me that you are more blind than a wraith.