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:
That might seem like it covers everything, but Vishnu still found a way to kill him:
Let's review how it gets around the conditions of the boon:
So as always, Vishnu triumphed in the end.
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
This is written in Srimad Bhagwatam 7.8.
Prahlada was challenged by his father in the following sentence:
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.
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.