Why does God show so much anger in the Old Testament?

This is a question that many of us may have asked when pondering the difference between the angry, wrathful and jealous God of the Old Testament and the loving and merciful God of the New Testament. 
 
But first, a little quiz. I’m going to provide a few verses from the Bible, without a saying which book they’re from and you guess if it’s from the Old or New Testament. 
 
“And thou, Capharnaum, which art exalted unto heaven, thou shalt be thrust down to hell.” 
 
“You brood of vipers, who hath showed you to flee from the wrath to come?” 
 
“He shall also drink of the wine of the wrath of God, which is mingled with pure wine in the cup of his wrath: and shall be tormented with fire and brimstone.” 
 
“Can a woman forget her infant, so as not to have pity on the son of her womb? And if she should forget, I will not forget thee.” 
 
“I have loved thee with an everlasting love, therefore have I drawn thee, taking pity on thee.” 
 
“But the mercy of the Lord is from eternity and unto eternity upon them that fear him: and his justice unto children’s children, to such as keep his covenant, and are mindful of his commandments to do them.”  
 
Give yourself a second and guess which Testament each verse is from. 
 
This may shock you, but the first three verses that I provided (talking about God’s wrath) are all from the New Testament, while the other three (talking about God’s love and mercy) are all from the Old Testament. 
 
So, what’s going on here? To put it simply, many of us have been taught a false interpretation of the Old and New Testaments. So, what is a more accurate reading then? First, we see God’s love very clearly in both Testaments. The Old Testament tell us about how God created man, how He made a covenant with His chosen people, how He promised us a Redeemer. The full extent of His love is shown in the New Testament. For example, it tells us that God is love, that He has prepared for us an eternal kingdom for those who follow Him, etc.   
 
Regarding His wrath in both Testaments, man deserves it. Here’s what I mean. We know of God’s wrath towards Israel in the Old Testament and how it entails Israel being conquered and sent into exile. The reason? They had turned away from God who loved them and broke their covenant with Him by worshipping idols. Remember that God liberated them from slavery in Egypt and they decided to worship lifeless statues instead. This idol worship of theirs also entailed gross sexual immorality and the sacrifice of their own children to those same idols. If you ask me, exile is a light sentence, all things considering!
 
At the same time, we always see that when God punishes Israel, there is always a mention of His mercy and how He will never forsake His covenant. He always comes back with a path of redemption, calling on Israel to repent of their sins and turn back to Him. 

The New Testament carries on this same theme. God loves us with an everlasting love. Consequently, he wants us to live with Him for eternity (that’s what heaven is, seeing God face to face). Also, when we sin, we separate ourselves from God, incurring His wrath (that what hell is, eternal separation from God). Nevertheless, while we are still on this earth, God has laid out for us a path for repentance (confession).
 
Thanks be to God for His everlasting love!