To kill Hiranyakashyipu, why did Lord Vishnu take on an avatar that was half human and half lion? He could have just come straight in his original form and killed that Daanav. Is there any specific reason behind this?
The reason that Vishnu went through all that rigmarole is described in the Srimad Bhagavatam. To avenge the death of his brother Hiranyaksha at the hands of Vishnu in his Varaha (boar) incarnation, Hiranyakshupu undertook long and arduous Tapasya (deep meditation), until Brahma appeared before him. He asked Brahma for the following boon:
O my lord, O best of the givers of benediction, if you will kindly grant me the benediction I desire, please let me not meet death from any of the living entities created by you. Grant me that I not die within any residence or outside any residence, during the daytime or at night, nor on the ground or in the sky. Grant me that my death not be brought by any being other than those created by you, nor by any weapon, nor by any human being or animal. Grant me that I not meet death from any entity, living or nonliving. Grant me, further, that I not be killed by any god or demon or by any great snake from the lower planets. Since no one can kill you in the battlefield, you have no competitor. Therefore, grant me the benediction that I too may have no rival. Give me sole lordship over all the living entities and presiding deities, and give me all the glories obtained by that position. Furthermore, give me all the mystic powers attained by long austerities and the practice of yoga, for these cannot be lost at any time. [emphasis added]
That might seem like it covers everything, but Vishnu still found a way to kill him:
As a snake captures a mouse or Garuḍa captures a very venomous snake, Lord Nṛsiṁhadeva captured Hiraṇyakaśipu, who could not be pierced even by the thunderbolt of King Indra. As Hiraṇyakaśipu moved his limbs here, there and all around, very much afflicted at being captured, Lord Nṛsiṁhadeva placed the demon on His lap, supporting him with His thighs, and in the doorway of the assembly hall the Lord very easily tore the demon to pieces with the nails of His hand.
Let's review how it gets around the conditions of the boon:
- Vishnu assumes the form of Narasimha, a half-man half-lion creature, so he's neither a human being nor an animal.
- He kills him on the doorstep, so it's neither inside nor outside.
- He kills him at twilight, so it's neither day nor night.
- He puts him on his lap before he kills him, so it's neither on the earth nor in space
- He kills him with his fingernails, which is dead matter, so neither a living thing nor an inanimate object.
So, as always, Vishnu triumphed in the end.
This is written in Srimad Bhagwatam 7.8.
Prahlada was challenged by his father in the following sentence:
"O most unfortunate Prahlada, you have always described a supreme being other than me, a supreme being who is above everything, who is the controller of everyone, and who is all-pervading. But where is He? If He is everywhere, then why is He not present before me in this pillar?"
He was, He is and He will be.
Hiranyakashipu, unable to control his anger, smashed the pillar with his mace, and then following a tumultuous sound, Vishnu in the form of Narasimha appeared from it and in defence of Prahlada went to attack his father, Hiranyakashipu.
In order to kill Hiranyakashipu and not upset the boon given by Brahma, the form of Narasimha was chosen. Hiranyakashipu could not be killed by human, deva or animal, Narasimha is neither one of these, as he is a form of Vishnu incarnate as a part-human, part-animal.
He comes upon Hiranyakashipu at twilight (when it is neither day nor night) on the threshold of a courtyard (neither indoors nor out), and puts the demon on his lap (neither earth nor space). Using his nails (neither animate nor inanimate) as weapons, he disembowels and kills the demon.
So, basically, it was done because of the boon Brahma gave to the demon because of his penance. If the mentioned steps were not taken, Brahma's boon would have broken, which wasn't intended.
Because King Hiranyakashyipu, who was father of little Prahlad, a great devotee of Lord Vishnu, was granted a boon from Brahma that "No human, No animal, No one in the skies, No one on the land" will be able to kill him.
Hiranyakashyipu's sins were increased to great extent that even Deva's were getting scared of him and couldn't stop him, so Lord Vishnu took avatar of half human, half animal, and he killed him in middle of skies and land.
Source: Wikipedia - Hiranyakashyipu
Once Lord Brahma had granted a boon to the demon Hiranyakshyap that any God,any Demon,any Human or any Animal cannot be kill him and he will not die in Earth or Space and he will not die in his home or outside and he will be not kill with any weapon and he will not die on day and night. That's why Lord Vishnu took his incarnation Narasimha for saving Hiranyakshyap's son Prahallad who was a follower of him and Narasimha was not a God,not a Demon,not an Animal and not a human,he was a half lion and half human and he had killed Hiranyakshyap not in his home and not in outside,he had killed him in the middle of his home and outside and he had not killed him with any weapon,he had killed him with his nails and he had not killed him in the Earth or Space,he had had kept him in his lap and killed him and he had not killed him on day and night,he had killed him on evening. This is the reason that why Lord Vishnu take on an incarnation that was half human and half lion.