There is an excellent talk on "why" the Atonement was necessary by the late Cleon Skousen:
A Personal Search for the Meaning of the Atonement Cronk
This same talk is available for purchase. Let me know, or just google it.
It boils down to this. God is a god of law. He, too, must obey eternal law, just as we must do in order to return back to Him.
But after the Fall of Adam and Eve - returning back to Father became impossible. Death and Sin were not things we could conquer, unaided.
So the Father sent His Son, Jesus Christ, to "atone" (make as one). Christ suffered so horribly in the Garden and on the Cross that the "intelligences" which comprise all matter were moved with compassion. Not for our sakes, but for His sake, we can be exalted and brought back into the presence of the Father again. Christ is our advocate. He can claim us as "His" - He earned the right to do this by the things He suffered.
Skousen does a much better job of explaining it than I do.
Consider this verse:
Mosiah 15: 9
9 Having ascended into heaven, having the bowels of mercy; being filled with compassion towards the children of men; standing betwixt them and justice; having broken the bands of death, taken upon himself their iniquity and their transgressions, having redeemed them, and
satisfied the demands of justice.
Who is demanding justice? It's not God. It's not Christ. They are all about mercy. The entire Plan of Salvation is merciful!!!!!
The demands of Justice are the intelligences which comprise the universe. The things that ACT (intelligences) and the things that are acted upon (element) are the building blocks of all existence.
Now look at this verse:
Alma 42: 24
24 For behold, justice exerciseth
all his demands, and also
mercy claimeth all which is her own; and thus,
none but the truly penitent are saved.
The Atonement does not prevent punishment, it reverses the effects. Christ said that our sacrifice is to be a broken heart and a contrite spirit (we must be penitent, willing to obey). The verse does not say "none but the truly PERFECT are saved" !!!! It says "none but the truly PENITENT are saved" !!!! It's not about perfect performance, but rather about our relationship with Christ.
Does this help any?