All you need to understand!! is to get the facts of the world since the beginning in one paragraph! Pretty hard to do.
The overall scheme of God is to have the people he created live in a world of abundance and safety, and after we live here a lifetime to live on with Him in heaven. When we harm each other (sin) we can't live here well, and God can't have sin in heaven.
The first bunch of people were failures, couldn't live well on this earth, and God couldn't have such people in heaven. They had no love for each other or for God. God sent the flood and started over, and this time he created a race of people to learn about Him, made Abraham the start of the race, and scripture gives the history of that. It is the OT. That is a teaching in learning to be kind to each other, and even accept any person regardless of race, that wanted to join in God's family.
God knew that when the people He created determined to be sinful, would never change from that, they had to be gotten rid of or the world could not be a happy, fruitful place. You are reading that as God being cruel. God is not cruel ever.
Christ is a theme all through all of history. Christ is part of God and God is eternal. Something supernatural happened through Christ that means that when we sin, we can still be counted as sinless. Our part in that is that we know there is a God who is holy, and we repent of sin. Repent means we turn from it. Because God is eternal, but God uses our awareness of time to reveal Himself to us, God had His Son live in the time we use. The world uses this to divide time into before Christ and after Christ. Before Christ lived on earth as a man this was called atonement, and a shadow of what was in eternal time was used of Christ, through symbolism of Him. They used the blood of animals as a shadow of the real blood of Christ, with priests, temples, many things called the sacrificial system. We are to study this system to better understand Christ, but we are not to use it after Christ lived with us. The NT tells us how to use the Lord Christ to live abundantly and free from sin using Christ as He lived and died as a man.
The main difference between the two testaments is not in God, but in how we are to use Christ for forgiveness of sin.