If the Supreme Lord is all-powerful, why should He try to reform His devotee by putting him in distress? The answer is that when the Supreme Personality of Godhead puts His devotee in distress, it is not without purpose. Sometimes the purpose is that in distress a devotee's feelings of attachment to Krsna are magnified. For example, when Krsna, before leaving the capital of the Pandavas for His home, asked Kuntidevi for permission to leave, she said, "My dear Krsna, in our distress You were always present with us. Now, because we have been elevated to a royal position, You are leaving us. I would therefore prefer to live in distress than to lose You." When a devotee is put into a situation of distress, his devotional activities are accelerated. Therefore, to show special favor to a devotee, the Lord sometimes puts him into distress. Besides that, it is stated that the sweetness of happiness is sweeter to those who have tasted bitterness. The Supreme Lord descends to this material world just to protect His devotees from distress. In other words, if devotees were not in a distressed condition, the Lord would not have come down. As for His killing the demons, or miscreants, this can be easily done by His various energies, just as many asuras are killed by His external energy, Goddess Durga. Therefore the Lord does not need to come down personally to kill such demons, but when His devotee is in distress He must come. Lord Nrsimhadeva appeared not in order to kill Hiranyakasipu but to save Prahlada and to give him blessings. In other words, because Prahlada Maharaja was put into very great distress, the Lord appeared.
From Srila Prabhupada's Krsna Book ch. 88: The Deliverance of Lord Siva