It was a different time and God wanted His people to be seperate and not do the things that the people around then were doing. When God says things like KILL, He has reached His limit of tolerance and His wrath is coming. God had the people killed because of the wicked things they were doing like: sex with animals (kill the animals), molesting the children which messes with their minds and when the molested grow up many of them will be molesters. Yes there were some that were not molested but God could not say (kill this one but let that one live). All children that were killed went to heaven because they did not understand. God is just.