It's funny. The Gnostic texts found in the Dead Sea Scrolls actually address that. In those texts, the god in the Old Testament and God in the New Testament are two different beings. The being in the Old Testament is the Demiurge, who was basically the antithesis of the true God. He's sort of what pop culture has wrongly made Satan out to be. In those texts, the Demiurge poses as God to corrupt mankind and make people believe that the physical world is all there is. Jesus was actually sent from the true God and his purpose was to redefine people's understanding of existence and to teach them how to rise above their physical limitations and get closer to the true God. There are a lot of other texts that build on those ideas too. For example, in the Gospel of Judas, Judas wasn't actually a betrayer. He was Jesus most trusted disciple. Jesus knew he had to die to fulfill his mission, which is something that true even in the accepted gospels. He entrusted Judas to make it happen and he also explained the truth of who he was to Judas, something that he didn't share with any of his other disciples.
Anyway, I think every Christian that actually reads the Bible has to question God a little bit when going between the Old and New Testament. The king in the New Testament was Jesus and he came delivering the Word of God, which teaches you to love your enemies. The king in the Old Testament was David and God empowered him to slaughter all their enemies indiscriminately.