It is the origin of sin. We went into a spiritual rebellion. Sin is not animal nature, God didn’t send Christ for the animals, it is a rebellion of our spiritual nature. Satan’s rebellion was sin, but God didn’t send Christ to redeem him. It does beg the question though, where did sin come from with a perfect God, how did imperfection originate in a perfect, omnipotent system? It seems the entire spiritual system is far more complicated than our human minds can understand and to try to constrain it to descriptions based on our limited experience is pretty flawed.
God allows us to sin because it would be wrong to deny us free will. If we choose to serve Satan and do evil, that's not God's fault, it's ours.