The Lord always honours His promises and this is seen in Him sparing Banasura's life, said M.V. Anantapadmanabhachariar in a discourse. Lord Krishna's grandson Aniruddha fell in love with Usha, daughter of the demon king Banasura. Banasura was a great devotee of Lord Siva and had been granted many boons by Siva. Siva's army stood guard over Banasura's kingdom. But in spite of this, Aniruddha was able to marry Usha secretly. When Banasura came to know of this, he was furious and he imprisoned Aniruddha.
Krishna then came with His army and fought those guarding Banasura's kingdom. Banasura had 1,000 hands, and Krishna began to cut off his hands, one by one. He was preparing to cut off Banasura's head too, when Siva intervened. Siva then told Krishna that He(Siva) had granted Banasura many boons, and the promise that He (Siva) had made to Banasura that He would save him must be honoured.
Siva also reminded Krishna of His own promise to Prahlada. Lord Narayana, as Narasimha, had promised Prahlada that he would not harm his descendants. That is why He refrained from killing Mahabali during the Vamana avatara. That being the case, how could he kill Banasura, who was a descendant of Prahlada? When Siva spoke thus to Krishna, Krishna stopped cutting off Banasura's hands. He had already lopped off 996 hands, but he spared the remaining four. He also spared Banasura's life. Thus Krishna made sure that Siva's promise to Banasura was kept, and also His own promise to Prahlada.
The Lord is keen that promises be kept. That is why He took so much trouble to kill Hiranyakasipu. He need not have taken the trouble to assume a half-man, half-lion form. He need not have taken care to kill Hiranyakasipu at a time that did not violate the request Hiranyakasipu had made to Brahma regarding the timing of his death. The Lord used His nails to tear Hiranyakasipu. He wanted to make sure that the boons granted by Brahma to Hiranyakasipu were not rendered useless. The Lord not only keeps His promises, He also makes sure that the words of His true devotees are honoured.