Just for a moment, think that you are God. Someone prayed you and asked to fulfill the wish that no man could kill him. First of all you told him to ask another boon as it was impossible. But he told that if you could give any boon, then only that the same. Otherwise he would go empty hand. So you had to give him the same. Now answer how can you finish him if he is doing evil and his death has been the need of time? Same happened with Lord Krishna.

Narakasur was a demon who had got boon from God that no man could kill him. Being arrogant with the power, he bothered everyone. He kidnapped about 16100 girls and made them captive. His death had been essential for the welfare of the world. So Krishna along with Rukmini {not Satyavama} marched for the war with Narakasur. When they were blocked by a demon called Mura, Krishna killed him {so Krishna is called Murare}.Thereafter a fierce battle was fought with Narakasur. Narakasur had been protected by God's blessing. So by God's power he could even injure Krishna. But suddenly Narakasur was attacked up to death by Rukmini. Narakasur shouted," Oh God, you deceived me" Lord Krishna told," It was not betrayal, Narakasur. You told no man could kill you. And you are not killed by man but by woman" Then Narakasur died.

Lord Krishna and Rukmini released all 16100 girls. But there was still a problem for those16100 girls as it was very strict society at that time. The girls kidnapped by demon could not get space in the society. So Lord Krishna built some palaces for them and provided them food, cloth and shelter.

Unfortunately, those 16100 girls were called the wives of Lord Krishna {but Mahabharat does not tell like that} which was not true at all. Even today in 21st century, if a man gives shelter to a woman, our society comments negatively. So there is no matter of surprise that Lord Krishna's mercy towards those 16100 girls was criticized in that way by the then society. There are many social servers in this world who have given help to many women by establishing many social organizations. Are those women wives of those social servers? If not we can't blame Krishna.
Discovered by +Don Prince