I am reading the Bible through these days, and I can say that I am a little surprised. I think God wants all the people, and that is why we are created. I don't understand why the Bible talks only about Israel, only about its people and its kingdom. In the Exodus, it is said that God killed every first male child of the Egyptians to let the Israel go. How can this happen, of course it can't be true. Why would God hate some of His people and love without conditions the some other people?
There are several issues you bring up that need to be addressed in this question, and it is going to take a little space.
You are correct in your assumption that God loves the whole world and not just Israel. However, Israel was, indeed, specifically chosen as part of God's plan to proclaim His name throughout the world.
Here is a general summary of the Bible.
Israel was just one part of God's plan to draw all mankind back to Himself. Israel played a vital role from about 2000 B.C. to about 33 A.D.. Although it only plays significantly for 2,000 years, it does comprise the vast majority of the Bible.
Still, God's choosing of the Jewish people was specifically to be a blessing to the entire world:
In the Jewish Temple itself, there was the Court of the Gentiles specifically designed for non-Jews to come and worship God.
Yet, God does not love Israel without condition. Time and time again, God judged Israel for their own rebellion against Him. Psalm 78 shows Israel's cycle of rebellion against God and God's judgment of them, and there are many instances of this.
In Numbers 21, God delivered a great victory for them, and shortly thereafter they began grumbling against God, doubting His promise and love and provision. As a result, God brought judgment upon them.
The Egyptians worshiped false gods of their own making who were no gods at all and actually worshiped the pharaohs themselves as gods. Worshiping false gods often degrades into horrific practices like the offering of babies as sacrifices. God desired to show the Egyptians that they were worshiping false gods, and He desired to draw them back into a relationship with Him. He did this by great signs and wonders, including 10 plagues. These 10 plagues were basically a showdown between God and the gods of the Egyptians--the god of the Nile, the god of the harvest, the god of the sky, etc., and finally the god of the line of the pharaohs. God won the showdown, and it is important to note that the whole purpose was for the Egyptians to know who God really is.
It is true, in fact, that God did kill every firstborn of Egypt, but this was not without warning. God righteously judged Egypt for their rebellion against Him, but He also provided a way to escape His judgment through the offering of the Passover Lamb. This provision was open to anyone.
God always does this. In the days before the flood, Noah proclaimed that judgment was coming, but only his own family responded and entered God's provision for escape from judgment.
So, God does not hate people. God loves all people and desires that all would return to Him. The fact that God judges does not mean that He has to hate. He upholds His own justice, but He does not hate.
God's Judgment and Mercy
So, God will always bring judgment, but He always offers a pardon to those who will accept it. The penalty for rebellion against God's authority, which is really high treason, is to be separated from God. God also has the right to take the life He has given when it is used in rebellion against Him. However, His desire is always that people would respond to His offer of mercy.
God the Son entered into the world, being named Jesus, and took upon Himself the penalty which we ourselves deserved for our rebellion against God. Because of this, He can righteously say that our penalty is paid and justice has been served. This is the whole essence of Christianity. Our penalty and debt is paid by another, if we will only accept that in faith. If we attempt to pay off our own debt or reject His offer, then we will indeed pay the penalty for our own rebellion.
Still, many absolutely reject this offer and do not want to live in relationship with Him. Someone put it this way... either we say to God, "Your will be done" and accept His offer of mercy, or God will say to us, "Your will be done" and give us our choice to live apart from Him.
God does, indeed, love all nations. The baton was passed, in a sense, from the Jewish people to the Gentiles in the early days of the church. To be sure, the first Christian missionaries were all Jewish, and there have always been Jewish believers whom God has used in mighty ways to take the message of salvation and forgiveness to all people.
The concept of all nations comes first in the first chapter of the book of Genesis:
This gets carried along with Noah:
We see it in Abraham's call as noted above. It is in the Psalms as well:
It is in prophecies of the Messiah:
It is in the Great Commission:
Finally, the prophecy is fulfilled in Revelation:
So, God loves all people and desires that all would accept His provision for the payment of their sin. He does not love some people without condition and hate others for no reason. In fact, He offers all people forgiveness of their sin, but He does not force anyone to love Him against their will. He does righteously judge those who rebel against Him, but always provides a way of escape.
I'll focus on the first question which relates to the identity of Israel. The Bible focuses on Israel, because Israel is the Church. The Bible is about God's creation of the world and His promise of redeeming His fallen creation through the Christ. In Genesis 3:15, after Adam and Eve sinned and brought death and evil into the world, God promises a Savior, the "seed of the woman." Then, God gathered together a people around this promise; this is the Church, God's people gathered by Him around the promise of the coming Christ.
The Church therefore began with Adam and Eve, continued through their son Seth, and then down the line to Noah. After the flood, it continued onward to Abraham, Isaac, and Jacob. Jacob was renamed Israel, and his descendants are the Old Testament Israel, the people of God. Yet, not everyone descended from Jacob is part of Israel. Paul makes this point in Romans 9:6-8:
"For not all who are descended from Israel belong to Israel, and not all are children of Abraham because they are his offspring, but 'Through Isaac shall your offspring be named.' This means that it is not the children of the flesh who are the children of God, but the children of the promise are counted as offspring."
The point is that it is not physical descent which makes a person a part of Israel, it is faith in God's promise of salvation through the Christ. The book of Hebrews picks up this point extensively (cf. Hebrews 11:1ff). In addition, people such as Rahab the prostitute of Jericho was incorporated into Israel, the Church, through faith in the Lord (Joshua 2:8-13). Ruth, the Moabite, was also incorporated into the Church through faith (Ruth 1:16-17).
So, it is faith in the Lord's promise of the Christ which makes a person part of Israel, the Church (cf. Galatians 6:16; Ephesians 2:11-22).
That's why the Bible talks about the people of Israel; the Bible is God's revelation to us of His actions in creating and then saving the whole world through Jesus Christ. God made this promise of salvation to all people, and this promise is believed in by the Church. So, in the Old Testament the Church is eventually called "Israel," but it is not just the physical descendants of Jacob who are "Israel;" indeed, not all of them are truly part of "Israel" as is seen, for example, in the days of Elijah the prophet (cf. 1 Kings 18:22ff).
The purpose of Israel, though, is not to horde God's blessings. Rather, Israel is meant to be a witness to God's promise of salvation through the Christ. God set Israel apart in the Old Testament to be this witness in order to call the Gentiles (i.e. the "nations") into the Church. Likewise, in the New Testament the Church is the New Testament Israel which continues this witness, this time in the light of the Christ who has died and risen and is returning to complete the restoration of all creation.
So, God loves all people and wants all people to be saved (1 Timothy 2:4). The purpose of the Church is to proclaim the Gospel of Jesus Christ to all people so that they may come to faith and thereby be incorporated into Israel for their salvation.
Short answer: The bible focuses on Israel because it was the mechanism that God chose to use to bring Jesus into the world in his bodily form and thus bring about atonement for every people group. The calling of Abram / Abraham that is the start of the line leading to Christ is placed right after the story of the tower of Babel (with a list of descendants in between) because it is the start of His ultimate answer which culminates on the day of Pentecost. Genesis 11 - God disperses the peoples, and in Acts 2 he calls them back together provisionally in the unity of the Church. In between these two events the main focus is on the family line that will result in Jesus being born on the earth.
There are as previously indicated much deeper reasons and metaphors built into the concept of Israel but the above is the first superficial layer.
I have read your question the answers and the comments over and thought I might answer the original question with my observations.
Why does the Old Testament talk mainly about the Nation of Israel?
If we take the Old testament by itself we do not see the purpose of those revelations. But on the other hand, when we take the Old Testament in relationship to the New Testament; what we find is that the Old Testament becomes a preamble to the New Testament. So what then, the you, ask is a preamble?
According to Merriam Webster the definition of a preamble is:
In the case of the Old Testament in relation to the New Testament, both of these apply.
The Old Testament is an introduction in which it give us the reasons that mankind needs a savior to begin with. Secondly It gives us an outline of God's overall plan to reclaim his creation up to the point of sending a savior.
If we contemplate the Old Testament in its entirety. What we find is that God tells us exactly why mankind fell out of his grace in the beginning, and then it goes into giving us a path whereby God assigns righteousness for the faith of the individual. The Old Testament is a compilation of God's grace, followed by man's rebellion, followed by God's grace, followed by man's rebellion. In the Old Testament God is giving us his plan whereby he makes covenants as he goes. All of which are dependent on Israel's unwavering devotion and worship of God.
The New Testament then can be thought of as the completion of the Old Testament. In that it gives God's ultimate solution and forgiveness.
The old testament is the groundwork for God's sending a savior into the world, the reason it talks about the nation of Israel is because that was his chosen nation; and by that what we mean is that it is the nation through which God made his salvation enter into his creation. Additionally, the old testament teaches us that even though our disobedience separates us from God, he loves us enough to save us, in spite of it.
The Bible as a whole then is God's plan of Salvation with the Old Testament being the why, and the New Testament being the how.