No god, man or animal could kill him during day or night. He could not be slain inside or outside the palace. He was thus highly puffed up with ego and wanted his subjects to worship him. His own son Prahlada was a great devotee of Vishnu and did not leave his devotion at any cost. The tyrant tried to kill his son. He wanted to know if the Lord of Prahlada was omnipresent; was he present within the column of the palace ? In great rage, he struck the column violently, whereupon, Vishnu came out of the column, tearing it off, vindicating the honour of His devotee.
He had assumed the form of a man-lion (who is neither a man nor an animal). He was killed in twilight (which was neither day nor night). He was killed at the doorstep (neither inside nor outside of the palace). See : Prahlada , Hiranyakashipu , Das Avataras (man-lion).
References :
1. Kohli,Surindar Singh ed,Dictionary of Mythological References in Guru Granth Sahib 1993