Davis began his career as a Brotherhood of Steel scribe. He is brilliant, although somewhat lazy. He knows that people of his caliber are in high demand and he takes advantage of the fact. He sees the Brotherhood as a means to an end and nothing more. He thinks it will sound good on his resume'.
Davis was not at all pleased when the war broke out with the NCR. Even though he knew he was part of a military organization, the thought of having to fight and getting killed in a war never crossed his mind. When he heard that there was going to be a desertion from the bunker, he quickly joined in. Unfortunately, when the time came to leave he was the only one who was caught. When questioned about his reasons for leaving he claimed he was disillusioned by the war and the tarnished record of the Brotherhood. He was ashamed to admit his real reasons.
Davis is not a coward. He simply wants to live and let live. He doesn't believe in war unless the enemy is on your doorstep or a proven threat. In such a case he would fight to defend his country. He just doesn't see the war with the NCR as being that type of war.