Why did Krishna forgive his 100 sins?

Shishupala was killed by Krishna during the Rajasuya Yagya conducted by Yudhishtir for his new kingdom - Indraprastha. But, why would Krishna kill someone during such a pavitr and holy event? Lets, start off by Shishupal's birth. 

Shishupala was born to King Damaghosha and Strutashuba (who was Vasudeva and Kunti's sister and thus Krishna's aunt) of the Chedi kingdom. He was born with a defect - he had 4 arms and 3 eyes. This scared his family so much so that they were ready to oust him from the house just when they were warned by heaven (an akashvani ) that when a certain person took the person on his lap, his superfluous body parts would disappear and that person would be the cause of his death. 

So, one by one, the worried parents called their relatives and kings of nearby kingdoms to check who held the reins of their child's death. Among the called were also Strutashuba's nephews Krishna and Balaram. As soon as Krishna placed Shishupala on his lap, his extra eye and hands disappeared indicating that his death would be at the hands of Krishna. 

The scared Strutashuba made Krishna promise her that he would forgive a hundred of his offences which he did promise.

During the Rajasuya Yagya, when the Pandavas decided to honour Krishna to be their guide in the path of dharma, Shishupal's blood boiled with rage for he hated Krishna. Thus, losing count of his sins, Shishupal started insulting Krishna, the Pandavas, Draupadi and even Pitamah Bheeshma. Consumed in his arrogance that Krishna couldn't kill him because of his promise, Shishupala lost count and after his 101st insult, Krishna decapitated him with his Sudarshana Chakra. 

His soul then liberated and attained salvation by merging into Krishna. To know why such a sinner's soul merged into Krishna, click here to read more. To know why Shishupala hated Krishna that much, click here to read more.