Why captivity, killing and that much blood in Old Testament? I know this is a symbol of removing all the sins in our life but still those were human beings?
The Apostle Paul says, "Oh, the depth of the riches both of the wisdom and knowledge of God! How unsearchable are His judgments and His ways past finding out!" (Romans 11:33). Mercy and justice are two important attributes of God. The atrocities committed by the inhabitants of certain cultures were beyond reprimand. Their hideous crimes included child sacrifices, burning and starvation of children, and utter forms of corruption, gross and lewd behavior, and relentless abuse. Yet, it is clear in the Scripture that God struck them after much patience and strict warnings to repent. By learning from these accounts in the Old Testament, we preview His wrath against sin and eternal judgment.
Look at the examples of Pharaoh, Rahab, Saul, and the Ninevities amongst many others. We witness God's mercy, patience, and willingness to give another, and yet another chance to repent. First, God stretches His arm with mercy and warning. Then, if repentance and efforts to correct their ways are scoffed at, He takes justice into His own hands and strikes with punitive action against vile and rebellious nations in order to save them and other nations from further savagery. Because the Amorites' sins had not yet reached their limit, God waited patiently, even if it meant His own people, who loved Him and believed in Him, would be subject to suffer severe oppression and also wait patiently for their deliverance for four hundred years as slaves (Genesis 15:13-16). No other nation other than the Canaanites was the children of Israel instructed to annihilate. This nation's vulgar practices were intolerable, and thus, their expulsion from the Promised Land required such stringent force. God's swift sword was more merciful than the slow and agonizing torture practiced by these cultures even upon their own children, upon whom, God spared by ending their young lives.
Consider God's mercy and justice exemplified in the passages below. His defeat of nations actually spared the horrific actions of such repulsive and wicked practices that had become the norm and moreover spread like a ferocious cancer or poisonous venom. Women and children were often spared and the Lord provided proper conduct of war (Deuteronomy 20). Therefore, such as in an aggressive terminal disease or a bite of a venomous snake, physicians sometimes opt to amputate the cancerous or damaged area so it does not destroy the entire body. Thus, God's intervention of justice is actually abundant in mercy. His intervention is like radiation to cancer and antivenom to a deadly snakebite. The treatment is necessary, powerful, and overwhelming, but the result is intensive healing, an extension of life, and a comprehensive renewal of quality of life.
“'Because they have forsaken Me and made this an alien place, because they have burned incense in it to other gods whom neither they, their fathers, nor the kings of Judah have known, and have filled this place with the blood of the innocents (they have also built the high places of Baal, to burn their sons with fire for burnt offerings to Baal, which I did not command or speak, nor did it come into My mind), therefore behold, the days are coming,'” says the Lord, “'that this place shall no more be called Tophet or the Valley of the Son of Hinnom, but the Valley of Slaughter.'" (Jeremiah 19:4-6).