Por que Morgoth amaldiçoou Hurin ao invés de matá-lo?

Provavelmente foi uma combinação de dois fatores:

• Vingança cruel Como DVK apontou em um comentário sobre a questão, o destino de Húrin foi uma tortura extremamente cruel; Morgoth amaldiçoou toda a família de Húrin e obrigou-o a assisti-lo (ênfase minha):

  Therefore Húrin was brought before Morgoth, for Morgoth knew that he had the friendship of the King of Gondolin; but Húrin defied him, and mocked him. Then Morgoth cursed Húrin and Morwen and their offspring, and set a doom upon them of darkness and sorrow; and taking Húrin from prison he set him in a chair of stone upon a high place of Thangorodrim. There he was bound by the power of Morgoth, and Morgoth standing beside him cursed him again; and he said: 'Sit now there; and look out upon the lands where evil and despair shall come upon those whom thou lovest. Thou hast dared to mock me, and to question the power of Melkor, Master of the fates of Arda. Therefore with my eyes thou shalt see, and with my ears thou shalt hear; and never shalt thou move from this place until all is fulfilled unto its bitter end.'

  ^{_{The Silmarillion III Quenta Silmarillion Chapter 20: "Of the Fifth Battle: Nirnaeth Arnoediad"}}

  Húrin tem que assistir a história de Túrin Turambar, uma das tragédias mais horripilantes do Silmarillion. E muitos dos principais atores são seus amigos e parentes.

• Ele ainda quer encontrar Gondolin. Parece razoável que Morgoth assuma que Húrin desejaria retornar a Gondolin depois dessa longa tortura, e pôs espiões em Húrin com esse objetivo em mente (grifo meu). ):

  Thus his freedom did but increase the bitterness of Húrin's heart; and he departed from the land of Hithlum and went up into the mountains. Thence he descried far off. amid the clouds the peaks of the Crissaegrim, and he remembered Turgon; and he desired to come again to the hidden realm of Gondolin. He went down therefore from Ered Wethrin, and he knew not that the creatures of Morgoth watched all his steps

  [...]

  For Húrin stood in despair before the silent cliffs of the Echoriath, and the westering sun, piercing the clouds, stained his white hair with red. Then he cried aloud in the wilderness, heedless of any ears, and he cursed the pitiless land; and standing at last upon a high rock he looked towards Gondolin and called in a great voice: 'Turgon, Turgon, remember the Fen of Serech! O Turgon, will you not hear in your hidden halls?' But there was no sound save the wind in the dry grasses. 'Even so they hissed in Serech at the sunset,' he said; and as he spoke the sun went behind the Mountains of Shadow, and a darkness fell about him, and the wind ceased, and there was silence in the waste.

  Yet there were ears that heard the words that Húrin spoke, and report of all came soon to the Dark Throne in the north; and Morgoth smiled, for he knew now clearly in what region Turgon dwelt, though because of the eagles no spy of his could yet come within sight of the land behind the Encircling Mountains. This was the first evil that the freedom of Húrin achieved.

  ^{_{The Silmarillion III Quenta Silmarillion Chapter 22: "Of the Ruin of Doriath"}}

A liberdade de Húrin também causa algum outro caos, como o assassinato de Thingol, que por sua vez leva à queda de Doriath e à separação de elfos e anões, mas não está claro se Morgoth sabia ou não que essas coisas aconteceriam. É certamente possível, caso em que fornece mais um motivo, mas não podemos ter certeza.