It's been popular for the past hundred years or so to say "The Old Testament God is a God of Wrath." Even the best of Christians seems to wriggle uncomfortably and feel unable to deny it. After all, there were all those genocidal purges when Israel went in and wiped out the Canaanites (men, women, and children) at God's command. Or when the Israelites began worshiping local idols or having sex parties, and God wiped them out with plagues or whatnot.
And then we have Jesus, prancing about in his white toga and gentle blue eyes (?), kissing the little children and petting donkeys. It's a conundrum.
Unless of course, you pay attention to two things.
First of all is historical context. As Americans we enjoy believing that all people everywhere are exactly the same, completely equal in all ways. But the truth of it is that what works on prissy American might not be as effective on say, a bloody viking warrior. If God was going to deal with whole nations and cultures in different eras of history, it's very likely that he would have to roll up his sleeves with thunder in his footsteps and lightning in his fists. The people in early Israel were an uneducated, recently enslaved collectivist group, who lived in a primitive world completely soaked in idolatry and bloodshed. It's probably beyond our ability now to make judgements on how God should have dealt with people in that brutal era of history. But we can judge the fruit. Did he successfully set apart a people for himself, the Jews? Do they survive to this day? Did they record his words and begin the process of revealing God to the world?
Secondly, we need to take a second look at the Old Testament. The real theme of the Old Testament is not wrath, but "'hesed" which is the Hebrew word for unfailing faithful love. The pages of the Hebrew scriptures from Genesis to Malachi are soaked with this word and concept--that God was faithful to his people to covenant with them, to bless them, to never abandon them no matter how much they deserve it. The deeper you dig into the Old Testament, the greater the wonder of God's goodness becomes. Job (and Habakkuk) are God's word to us that doubt and questioning are natural in the life of faith. Habakkuk the prophet cries out "How long, O Lord, must I call for help, but you do not listen?" As Global University's Old Testament Survey points out, Habakkuk's honesty teaches us that "believers do experience doubt. Doubt is not a sign of a lack of faith" (p.184). Hosea marries a prostitute and remains faithful to her even when she continues sleeping around on him. Even take a grim book like Judges. It shows how willful and rebellious the people were, and even though God would allow them to be conquered by oppressive nations when they turned from him, he was faithful to raise up Heroes of faith to deliver them when they repented. The more one studies the Old Testament, the greater will be the deep impression of God's amazing patience and 'hesed towards the people of Israel, and through them, to the pagan world at large. (Ever read the book of Jonah?)
On a final note, the Jesus of the New Testament is exactly the same God represented in the Old. Jesus compared his second return to a master and warned of being an unfaithful servant: "The master of that servant will come on a day when he does not expect him and at an hour he is not aware of. He will cut him to pieces and assign him a place with the hypocrites, where there will be weeping and gnashing of teeth." As you study the life of Jesus a certain fear of his perfect veracity and righteousness will come on you. And don't forget the New Testament's portrait of Jesus in Revelation, with a sword coming out of his mouth, riding the white horse in Judgement on the whole earth.
The God of the Old and New Testaments is one and the same. At times he gave different strokes to different folks, as their time and place needed. But he himself is terrible in purity, perfect in holiness, and faithful in love. Jesus Christ is the same, yesterday, today, and forever.