Why did God completely destroy evil things in the Old Testament?

Many times in the Old Testament we see God either destroying evil Himself or telling Israel to completely destroy evil nations.  From a New Testament standpoint, it seems as if God is being cruel and unforgiving, but we must look at it from the context of the Old Testament.  God gave everyone a chance to stop their sin and turn to Him, but they refused (Genesis 15:16; Genesis 6).  Since these people would not repent, God did not want the Israelites to interact with them and get caught up in their sin.  This is why God told them to completely destroy these nations, so they would not fall into the trap of idolatry, which they did because they failed to follow God’s command (Book of Judges).  God promised not to destroy the whole earth with a flood again due to the evil of man (Genesis 9:11), but He still used various means to punish those who clung to evil and loved sin rather than righteousness.