OneGod your question is one of the most asked questions by anyone, Christian, atheist or whoever and it deserves an answer.
Believers explanation is a good one, but there is more to it that needs to be explained and I think it will help answer you questions. So let me try
Most people start their search of the Bible by looking at Genesis, when infact, in other books of the Bible, Biblical history actually is recorded that precedes creation, and this is where you have to start your search for why the crucifixion was necessary.
The reality is, creation did not start at the beginning of time as we know it. Creation had been going on for innumerable years before the creation of the earth. How do I know this? Because the Bible speaks of God as infinite, without beginning and end, and angels and demons were created before mankind. We tend to think salvation starts and finished with man in mind, and there is a sense it does, but there is another drama in play, concurrently with the fall and salvation of man.
The Bible describes God as an eternal king. Being infinite, He is everywhere. So, at every time and place, in all the vast reaches of the universe, God has been and is in full control. It is essential to recognize that His soverignity and Godhead has never been compromised. His work of creation, with all the apparent risks involved, was the work of his soverignity.
God in the past before the creation of the earth and man, had created the angels, and placed them in a hierarchy assigning them levels of responsibility and spheres of service. One archangel with striking wisdom and power (Ezekial 28:12-17, Jude 9) was named Luckifer and given a throne from which to rule over all the activities of the angels.. kind of like a prime minister (Isa 14:12-14).
We don't know how long this harmonious relationship in the heavenlies continued.. the Bible does not speak of it. Even the angels were endowed with freedom of choice and the crucial test of any creature, was then and remains now, their allegience to the will of God.
At some point in the time before creation of the earth, Lucifer was faced with a choice of his own. He shifted his gaze off God and onto himself and His own God given splendour (Isaiah 14:14), and declared himself to be "like the most high God."
In that moment of decision, he removed himself from the stabilising axis of God's will and began the swirling catapult into the oblivion of a godless being. His decision was final and never repented of.
Lucifer however was not alone in this choice. Rev 12:4-7 declares that a third of all the angels, joined him in this choice, perhaps an indication of his great leadership ability. With these fallen angels, he formed a kingdom of his own, a counterfeit kingdom of darkness. His name was changed to Satan (adversary) in keeping with his behaviour.
Now an important question. If God is soverign, why did he not just destroy Satan? Destroy all the angels who had fallen and formed their own conterfeit kingdom?
The only logical answer is that GOD ALWAYS HAS A PLAN. One day, there will be complete and total punishment for them who we now call demons, but in the meantime, He who is soverign is using them to accomplish another of His plans and purposes. The irony of His soverignity is that oneday they will end up worshipping him again. He has limited their power but allowed them the right to operate their kingdom of darkness and is an alluring option for all moral creatures in their exercise of moral freedom.
But how does all of this apply to us as created humans?
The scene here sets the scene for the situation at creation. After the fall of Satan, God began another creation.. the creation of man. He likewise endowed this being with freedom of choice, a dangerous step, considering the angelic response to this freedom.
Freedom of choice was and is essential to human personality. If man and woman were to be created in the image of God. His prime purpose in creating man, was to reproduce himself in human personalities, especially his traits of love and holiness. And these divine characteristics can only grow in the soil of moral freedom. Fellowship involves moral choice.
By this freedom, God sought to establish man and woman in a wholesome relationship to his soverignity. He sought to relate to them by love and not coercion.
He created Adam and Eve co-rulers over the earth and it's creation, and as an initial test, he placed before them a tree that they were forbidden to eat from "the tree of the knowledge of good and evil". (Gen 2:17). They were given a clear choice of compliance or disobedience. The tree was not a trap, or a teaser but was an inevitable test of their allegience. He knew Satan would come and tempt them... He is God and all knowing. This was no mistake on the part of a loving God, which would see millions head towards the same eternal damnation that the demonic angels were heading. In choosing to listen to the serpent, and not to continue on in their allegience to God, they had made a deliberate choice in their free will. In their decision, they failed, and fell, along with the angels. What if they had chosen the alternative, and resisted Satan, would God have allowed them to eat of the "tree of life" (the other forbidden tree in the garden) and be eternally confirmed in righteousness? Gen 3:24, Rev 22:2)
Now, did God fail to consider the consequences of the fall? Man was trusted with control over the entire earth, yet he failed on one piece of fruit? This entire venture, seems on the surface counterproductive and risky and sees the damnation of millions of created humans.. but the story continues and I'll pick it up in my next post.