War in the Bible

There are lots of verses in the Old Testament telling about war and how Israel fought their enemies, and how they treated them, I will give here only 2 verses:

When thou drawest nigh unto a city to fight against it, then proclaim peace unto it. 11 And it shall be, if it make you  answer of peace, and open unto you , then it shall be, that all the people that are found therein shall become tributary unto you , and shall serve you . 12 And if it will make no peace with you , but will make war against you , then thou shalt besiege it: 13 and when Jehovah your God delivereth it into your hand, thou shalt smite every male thereof with the edge of the sword: 14 but the women, and the little ones, and the cattle, and all that is in the city, even all the spoil thereof, shalt thou take for a prey unto thyself; and thou shalt eat the spoil of thine enemies, which Jehovah your God hath given you . 15 Thus shalt thou do unto all the cities which are very far off from you , which are not of the cities of these nations. 16 But of the cities of these peoples, that Jehovah your God giveth you  for an inheritance, thou shalt save alive nothing that breatheth; 17 but thou shalt utterly destroy them: the Hittite, and the Amorite, the Canaanite, and the Perizzite, the Hivite, and the Jebusite; as Jehovah your God hath commanded you ; (Deuteronomy 20:10-17)

There are many other verses in the Old Testament showing how the Israelites dealt with the gentiles as in 1 Samuel 15 or Numbers 31. Some Christians try to justify this to be tribe wars, and it was only a defensive case. However, the verses are very clear. According to the Christian perspective, why does the law tell Israelites to come and fight cities, and if they accepted peace, they are all taken as slaves? And why does God order the Israelites to take a part of land (the holy land) from those who were living in it? Where is the freedom of choice? Yes, Muslims believe that God gave Palestine to Israelites (at their time only, but when they broke God’s covenant, they have no more right in it), but there are some Christians who are treating with double standards. Why did God order to kill women, children, even the suckling as seen in 1 Samuel 15? Then we see Jews and Christians come and accuse Islam with barbarism? How come?