Because the bible is also full of instructions from "god" to make war on and kill unbelievers. "Convert or die" was how Christianity managed to spread in the first place- at the point of a sword or the flames of the pyre.
After the 10 commandments were handed out the rest of the laws were given and in them was also the order to kill every Canaanite, man woman and child. So who knows how that law applies except to personal murder, but genocide is OK.
The historical evidence is quite clear: religion is not the primary cause of war. If religion can't be blamed for most wars and violence, then what is the cause? The principal trigger is the same thing that produces crime, cruelty, loss of life, and other such things: "From within, out of the heart of men, proceed the evil thoughts, fornications, thefts, murders, adulteries, deeds of coveting and wickedness, as well as deceit, sensuality, envy, slander, pride and foolishness. All these evil things proceed from within and defile the man" (Mark 7:21-23). The problem, according to Jesus, is sin. James (naturally) agrees with Christ: "What is the source of quarrels and conflicts among you? Is not the source your pleasures that wage war in your members? You lust and do not have; so you commit murder. You are envious and cannot obtain; so you fight and quarrel" (James 4:1-2).. Sin is the #1 cause of war and violence, not religion, and certainly not Christianity. http://www.compellingtruth.org/religion-cause-wars.html
OK, first off, the the sixth commandment says to not kill as in murder. The Bible instructs us that God in His perfect wisdom is ultimately the judge of others and we are not.'Vengeance is mine, saith the Lord.' We are not to kill in such a way as to take vengeance on another person. If the Lord commands it, then that's different. That's a hard teaching, because why would God want His own creation destroyed? However, we can understand having to dispose of that which is completely corrupted. You must understand that the nations in ancient times that God commanded the Israelites to destroy were unrepentantly, absolutely corrupted. They regularly practiced acts such as ritual prostitution and child sacrifice. However, since those times, there have also been many wars fought in God's name that He did NOT command. God has dealt/will deal with those people.
unfortunetly god also gave us free will. and though they may think they are killing in his name. it does not please god that they do it. just like in times of war we fight to protect our country or a father fights to protect his family. would you not kill for your child? I believe in God and I can not say for sure whether or not I would or wouldn't but I know that I would try my best. you stand up for who you love and sometimes in doing so people put themselves in bad or unchristian places. but people must also remember that because one or 15 do does not mean that the whole group of the does or feels the same. I hope I make some sense