I understand what you are saying here. How is God telling the Israelites to put a city to the sword any different from a jihadist thinking God had told him to kill people, for example? But there is a difference; in the conquest of Canaan, some cities were burned up, such as Jericho (Joshua 6). This only applied to the cities in what was intended to be a 'holy land', who wouldn't make a treaty with Israel. The idea was that there would be a land where God's law was perfectly followed (a bit like the Islamic vision of a Caliphate) that would eventually be spread throughout the world. Everything in these cities was said to be 'devoted to the Lord' (v.17). By totally destroying something or someone, that thing was irrevocably God's, going up to heaven in smoke. The burnt offering described in Leviticus carried the same idea. However since that time, God has sent Jesus to be a perfect, once-for-all offering, so that there is no need for sacrifices any more. Because jihadists reject this they still live in the days when sacrificial practices were carried out to gain God's forgiveness.