Yours is a very common misconception of both the Old and the New Testament. There are two misconceptions here. First, it is common to view the portrayal of God in the Old Testament as angry and vindictive. This could not be farther from the truth. Second, it is a very common misconception created by evangelical Christianity that the New Testament presents a God who is all love and no judgment. I would like to challenge both ideas.
First, there is the God of the Old Testament. Of course, the prophets in the Old Testament teach "repent or perish." But they also show the absolutely amazing patience, love and grace of God. Let us consider a few examples. Jonah was very angry at God. Why? Because "I knew that you are a gracious and compassionate God, slow to anger and abounding in love, a God who relents from sending calamity." (Jonah 4:2) God was willing to forgive even the Assyrians their sinful behavior if only they would repent. Think about how stubborn and rebellious Israel was, yet God was patient, not wanting anyone to perish, but all to come to repentance and salvation. I could give an almost unlimited number of examples in the Old Testament of God’s love, patience and grace. There is Hosea. Here God uses the life of the prophet as a parable of how God feels about his people. He asks Hosea to take on an unfaithful wife, yet forgive her. God told Hosea, "Go, show your love to your wife again, though she is loved by another and is an aduleteress. Love her as the Lord loves the Israelites, though they turn to other gods and love the sacred raisin-cakes." (Hosea 3:1) I do not mean to imply that there is no godly anger or judgment in the Old Testament. Justice and judgment are very real in the Old Testament, but it is God’s deepest desire to show love and blessings to his people and even to the Gentiles. I think one reason people get the false impression that the picture of God in the Old Testament is more "angry" is that God dealt with his people in more physical ways, whereas in the New Testament the judgment is more on the spiritual level.
Then there is the portrayal of God in the New Testament. From your question, I can tell that you are aware that the New Testament shows God’ s patience, love and grace. There is an almost unlimited number of passages in the New Testament, as there so many in the Old which show God’s grace and love for his people. Unfortunately, there is a tendency in evangelical Christianity to present only this aspect of God. However, if you take a fair look at the sayings of Jesus or the other books of the New Testament, you will see a lot of emphasis on judgment and the righteousness of God. Like the other prophets, Jesus said, ""But unless you repent, you too will all perish." (Luke 13:3) Despite the false impression many of us are given because of the unbalanced teaching in many churches, Jesus taught as much about hell as he did of heaven. I see a lot about judgment in the book of Revelation. Think about Hebrews 10:26-31. This is arguably the most disturbing passage in the entire Bible. "It is a dreadful thing to fall into the hands of the living God."
You will have to decide for yourself, but I believe that the picture of God in the Old and the New Testament is essentially the same. This idea that the picture of God is radically different is mainly due to a misconception of what is found in the Old and New Testament; augmented somewhat because of the fact that the Old Testament is more about physical and the New Testament is more about the spiritual implications of rebellion versus repentance.
John Oakes, PhD