Old Testament /Ten Commandments/ add more O.T stuff here

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I am trying to piece together things, from when the Ten commandments were introduced , to books in the bible like Judges and the " wrath of God "..

Ten Commandments, Thou shall not kill, // I have recently been discussing T.O.T with a friend, and seems that things then were getting at one God pursuing humanity , and a people wanting a king and also forgetting their covenant with God and thus having to atone or pay for that in some way.

So if the parts of the Old Testament dealing with the Israelites going to war, and wiping out cities that were pagan, etc, and doing this for God, or because they were instructed by God.  How are people, scholars, or the church comparing that with when Moses brought the Ten Commandments ? More over my question is  are we really to believe that God  told these people to go to war and so forth.  * I am speaking in generalities because at the moment i can not recall exact books in the old testament, but there are plenty of stories in this regard *

Also in the book of judges alone, there are some rather disturbing stories, which are not for beginners in the faith i would say nor for children. An i have a hard time trying to make any sense out of the old testament and so i tend to gravitate more towards the New Testament in general.  

another thing,,

In the old testament, we hear about not so much about Christ, but the prelude to Christ and how Christ is the Word of God.. Did the people of the Old Testament only believe in God the Father at that time, meaning that it was only just God the Father starting out, and that there was no Trinity until we get into the new covenant ?  Or was the Holy Trinity always in existance but the people of the O.T were only worshiping God the Father and waiting for God the Son and not even really considering God the Holy Spirit ?