[41a] How can one reconcile the vindictive God of the old testament with the forgiving God of the new testament?

The old testament is the story of Jehovah of Armies. A God who wiped out the entire human race with the exception of 8 souls in an ark. 

A God who gave Moses the law of soul for soul and eye for eye. 

A God who permitted the killing of a slave to go unpunished so long as it was not immediate.

18 And in case men should get into a quarrel and one does strike his fellow with a stone or a hoe and he does not die but must keep to his bed;
19 if he gets up and does walk about out of doors upon some support of his, then the one who struck him must be free from punishment; he will make compensation only for the time lost from that one's work until he gets him completely healed.
20 And in case a man strikes his slave man or his slave girl with a stick and that one actually dies under his hand, that one is to be avenged without fail.
21 However, if he lingers for a day or two days, he is not to be avenged, because he is his money (Exodus 21).

A God who had 10 cowardly spies beaten to death in front of his face...

37 Yes, the men bringing forth the bad report about the land will die by the scourge before Jehovah (Numbers 14).

A God who defeated at least 9 nations in order to give the sons of Jacob the land of Israel. there being the Egyptians, the the Amalekites, the Hittites, Girgashites, the Amorites, the Canaanites, the Perizzites, the Hivites, the Jebusites (Deuteronomy 7:1 etc).

A God who permitted the conquering, enslavement and exile of the 10 Northern Kingdom tribes of Israel by the barbaric Assyrians under King Shalmanezer, who would lead their captives by nose rings and flayed them alive...

9 And it came about in the fourth year of King Hezekiah, that is, the seventh year of Hoshea the son of Elah the king of Israel, that Shalmaneser the king of Assyria came up against Samaria and began to lay siege to it.
10 And they got to capture it at the end of three years; in the sixth year of Hezekiah, that is, the ninth year of Hoshea the king of Israel, Samaria was captured.
11 After that the king of Assyria took Israel into exile in Assyria and set them down in Halah and in Habor at the river Gozan and in the cities of the Medes,
12 over the fact that they had not listened to the voice of Jehovah their God, but kept overstepping his covenant, even all that Moses the servant of Jehovah had commanded. They neither listened nor performed (2 Kings 18).

A God who permitted the destruction of Jerusalem by the Babylonian King Nebuchadnezzar and the enslavement of all his people in exile in Babylon for 70 years... 

8 And in the 5th month on the 7th [day] of the month, that is to say, the 19th year of King Nebuchadnezzar the king of
Babylon, Nebuzaradan, the chief of the bodyguard, the servant of the king of Babylon, came to Jerusalem.
9 And he proceeded to burn the house of Jehovah and the king's house and all the houses of Jerusalem; and the house of every great man he burned with fire.
10 And the walls of Jerusalem, all around, the entire military force of Chaldeans that were with the chief of the bodyguard pulled down.
11 And the rest of the people that were left behind in the city and the deserters that had gone over to the king of Babylon and the rest of the crowd Nebuzaradan the chief of the bodyguard took into exile (2 Kings 25).

A God who would not allow his prophet Moses to enter the promised land due to one mistake made in a 120 year life...

Whereas the new testament is the story of Jesus Christ. 
A son who suggested that we should turn the other cheek.
A son who cured the blind and the lame because he wanted to.
A son who did not lose one of his sheep other than, Judas, the son of destruction.
A son who commanded his disciples not to attack their enemies but to love them.
A son who defeated the whole world without killing anyone.
A son who was beaten for our sins without complaint.
A son who gave all of us the opportunity to enter not the promised land that lies between the Nile and the Euphrates, but the Kingdom of God that lies between 2008 AD and 3008 AD.

How then can it be true to say of God's son, Jesus, that...

3 He is the reflection of [his] glory and the exact representation of his very being (Hebrews 1)?

15 He is the image of the invisible God, the firstborn of all creation (Colossians 1)?

Is God a schizophrenic? The simplest answer to write to this question is go and see the Mel Gibson movie - Ransom. In that movie a nice loving highly motivated father becomes a violent scheming maniac in order to rescue his son who had been been kidnapped.

What lengths will a loving father go to in order to rescue his children from a depraved kidnapper? For Satan kidnapped Adam and his sons and help them to ransom for 4025 years from 3993Nisan14 BC when Adam sinned, until 33Nisan14 AD when Jesus ransomed him by dying on the torture stake. During this time God acted within the law, but often took no prisoners in his quest to rescue and ransom his children. God had a right to kill any son of Adam, who had not been born again, at any time, since they were judicially condemned to death by Adam in any event and were only living by virtue of the mercy of God who granted a stay of execution to Adam and his sons. The whole post edenic system existed by virtue of God's mercy and Jesus' agreement to ransom Adam, although since Adam was always going to fail his test, that mercy was an intrinsic part of God's plan.

In life there are two things that a Christian and any good man has to do. He has to love his family and friends and pursue good works himself yes, but that is not sufficient. He also has to stand up and fight against bad. We all know this. It is a feature of this system in which we gain knowledge of Good and Bad as symbolised by the tree that our parents ate from. From the old testament is our knowledge of bad, and from the new testament is our knowledge of good. Form the law of Moses came the accurate knowledge of sin, but from the law of Christ we see the accurate knowledge of love. 

Yet the good book says...

6 for whom Jehovah loves he disciplines; in fact, he scourges every one whom he receives as a son (Hebrews 12).

Scourges are really painful. But at the end of the day God does not want to scourge anyone, he wants to love them all, like any father would. And he wants them to love him and each other. But scourging is a necessary part of the learning process which brings free willed intelligent beings into that love. It is as we said in I25a. In the old testament God was forced into playing a role that he would have preferred not to play, and in the new testament he is not playing any role at all. Rather he is portraying, through Jesus Christ, everything that he truly and genuinely is. And if you love him you will see that, and if you don't, you won't.

But is not the definition of love that the lover should do something he would prefer not to do for the sake of the one that he loves?