We should ask ourselves some questions Hastings. Why was Adam and Eve not slain the moment that they sinned in the garden? It was because "of the Lamb slain from the foundation of the world." Revelation 13:8. They were spared by the immediate stepping in of Christ to take their place when the plan was put into motion. Yes it already existed but it was begun at that very moment. When Adam and Eve sinned they plunged the human family into condemnation but (as a coin has two sides) the sacrifice of our Lord (though not made until Calvary) saved all of humanity and gave them a second probation.
Romans 5:18 "Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life."
The key here is the fact that scripture says "all men." I'm sorry if this is something that you already know but I tried to answer your question as best as I could.