Lord Narayana assumed the form of half-lion, half-man, and appeared as Narasimha to save Prahlada. Though we say He came to kill Hiranyakasipu, and this killing did take place, the proper way of putting it is to say that He came for the sake of Prahlada.
The Lord always keeps His word, and ensures that the promises of His true devotees are kept too, said M.V. Anantapadmanabhachariar, in a discourse.
Why did He have to assume the form of half-man, half-lion, to kill Hiranyakasipu? He could so easily have done it from His divine abode. But Hiranyakasipu had obtained a string of boons from Brahma. As a result, he could not be killed by man or beast, at night or day, on the ground or in the sky and so on. So in order to ensure that Brahma’s promises to Hiranyakasipu were not broken, the Lord had to assume the form of Narasimha. The Lord’s nails became His weapons. He put Hiranyakasipu on His lap and tore him up.
In reply to his father’s query, Prahlada had said the Lord was everywhere. Hiranyakasipu was going to strike a pillar, but there was no knowing which one he would strike. The Lord was therefore waiting in readiness to emerge from any pillar which the demon struck, for he was keen on making sure that Prahlada’s words were kept. When Hiranyakasipu finally did strike a pillar, the Lord emerged from it, roaring in anger at the demon’s treatment of His devotee — Prahlada. Is there anything more amazing than a pillar yielding such a form as Narasimha?
The Lord’s anger against Hiranyakasipu continued unabated even after He had slain the demon. Even Goddess Lakshmi did not approach Him. Though it may be assumed that it was His anger that kept Her away, the reason She kept away was to show the world the greatness of Prahlada. After all, the Lord had come to save the little boy, and shown the world that He was ready to help His devotees. It would, therefore, be proper to let Prahlada himself be responsible for calming the Lord and showing the world that He is moved by His devotees’ bhakti.
The Lord rages at ill-treatment of His devotees, and is calmed by their devotion.