I think a lot of people–including me when I was younger–think that there are two separate Gods in the Christian Bible. First there’s the mean vindictive God of the Old Testament, and then there’s the nice guy Jesus in the New Testament.
However, you do see all of Jesus’ character traits in the “God of the Old Testament.” For example, I grabbed the following from Baker’s Theology of Exodus:
On this occasion, Moses receives a special revelation of Yahweh’s character. He requests a look at God’s glory (33:18 ). The audacity of the request is overlooked, and Yahweh promises to reveal all his “goodness” ( 33:19 ). Whether the “glory” and the “goodness” are the same is not explained. But when Yahweh passes Moses on the mountain, six words or phrases are proclaimed that provide one of the fullest descriptions of the Lord’s character, no matter whether glory or goodness. Yahweh is compassionate, gracious, slow to anger, abounding in steadfast love, abounding in faithfulness, and forgiving ( 34:6-7 ).
According to the Bible, Jesus and the God of Abraham are the same. (Jesus said, “[B]efore Abraham was, I AM.”)
I have said this before, but the way I interpret the “two different personalities” is that in the Old Testament, you see God in His role God Almighty, creator of the heavens and earth. So yes He sends plagues and orders massacres, but you must keep in mind that He is in control of everything that happens and whether you die from a heart attack, an avalanche, or Joshua’s sword, in a sense “God killed you.” In this setting, God taught us how to live by issuing a long string of commands and giving rewards and punishments accordingly.
However, in the gospel accounts, God has become a man in order to teach us how to live by showing us through His own personal example. He didn’t contradict the Law, but fulfilled its spirit. He showed us how to understand it at a much deeper level. And since He had became one of us, He acted the way a perfect human would act. Jesus in His role as one of us didn’t use violence to resist evil.
If you object that this is all really complicated, yes I agree, but on the other hand if the Bible account were true, then you would expect it to be a bit difficult for a mortal to comprehend.