if God simply chose to ignore sin and save everyone anyway, God would be acting unjustly, which is in direct violation of His own nature. God cannot ignore sin. God can delay punishment, but in the end, justice must be served before any love can be shown.
People often ask how a loving God can send people to Hell. It is easy to look at the situation backwards from a human-centric point of view, but in reality, God had NO RIGHT to show us any mercy, yet he did anyway. Satan got swift and immediate justice for his rebellion against God (eternal condemnation with no hope of redemption) but we got a way back to God. By right, we should have received the same punishment that Satan got, but God chose to show mercy because He had already demonstrated justice.
God knew that Adam would fall, but God allowed it to happen because it would provide an opportunity for the world to see the extent of God's mercy rather than simply demonstrating justice again.
Over all those thousands of years before Christ, God was delaying the punishment of those who had already died so he could take it on Himself instead.