While Eru does take the good side e.g. Isildur, he doesn't make it easy for them. Maybe letting the Witch-King bind mortal spirits to the downs was his way of making things tougher for the people of Arnor. Remember that Melkor used up much of his original power twisting the peoples of ME to his own control, I find it hard to believe that a Numenorean wraith could keep mortal spirits from Mandos against the will of Eru.
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Fearlessness is better than a faint-heart for any man who puts his nose out of doors. The length of my life and the day of my death were fated long ago.
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