This is a common misconception. First, God is unchangeable in His being. He is neither more nor less loving in the Old Testament than in the New. Nor is He more or less just in the Old Testament than in the New. So as soon as we start talking this way, we are laying a charge against God that exposes our ignorance of God and the Bible.
God’s judgment on individuals for their sin is no less severe in the New Testament. For example, in Acts 5 God strikes Ananias and Sapphira dead because they lied about the price of a piece of land. At the end of Acts 12 God strikes Herod dead because of his sin. So God’s wrath is on display on a number of occasions even during the great days of revival in the early church after Pentecost.
Furthermore, if you think God’s wrath against nations is more severe in the Old Testament than the New, then you need to read all the way through to the end of Revelation. One thing becomes abundantly clear, God will be glorified in His judgement upon unbelieving sinners just as much as in saving sinners.
Armen