As Lazy Luber pointed out, Lord's anger was temporary .
His form is described as wonderful as well in Bhagavatam:
adṛśyatātyadbhuta-rūpam udvahan -- wonderful form
dṛṣṭvā taṁ mahad adbhutam
As soon as Prahlada approached Lord, he became very peaceful..
When Lord Nṛsiṁhadeva saw the small boy Prahlāda Mahārāja prostrated at the soles of His lotus feet, He became most ecstatic in affection toward His devotee. Raising Prahlāda, the Lord placed His lotus hand upon the boy’s head because His hand is always ready to create fearlessness in all of His devotees.
Thus, Prahlada says that this form is to be remembered to become free from fear:
ad yaccha manyum asuraś ca hatas tvayādya
modeta sādhur api vṛścika-sarpa-hatyā
lokāś ca nirvṛtim itāḥ pratiyanti sarve
rūpaṁ nṛsiṁha vibhayāya janāḥ smaranti
My Lord Nṛsiṁhadeva, please, therefore, cease Your anger now that my father, the great demon Hiraṇyakaśipu, has been killed. Since even saintly persons take pleasure in the killing of a scorpion or a snake, all the worlds have achieved great satisfaction because of the death of this demon. Now they are confident of their happiness, and they will always remember Your auspicious incarnation in order to be free from fear.
My Lord, who are never conquered by anyone, I am certainly not afraid of Your ferocious mouth and tongue, Your eyes bright like the sun or Your frowning eyebrows. I do not fear Your sharp, pinching teeth, Your garland of intestines, Your mane soaked with blood, or Your high, wedgelike ears. Nor do I fear Your tumultuous roaring, which makes elephants flee to distant places, or Your nails, which are meant to kill Your enemies.