Why did the old Adam have to die?
The old Adam had to die so the believer could be restored to a right relationship with God. Because Adam's sin made the old sin nature the only nature we inherit, it had to be crucified in the Spirit when we believe the Gospel, allowing God to justify us and impart a new divine nature.
Les teaches that when we become believers, the old sin nature—the old Adam we are born with—had to be crucified. He had to be literally put to death in the realm of the Spirit, and that is done when we believe the Gospel and Christ reckons it as crucified. Since Adam sinned, that old sin nature became the only nature, the one we are born with. In order to come back into a right relationship with God, that old Adam has to die; he has to be put to death by crucifixion, accomplished in the Spirit by God when we believe the Gospel. Then God immediately places opposite that old Adamic nature a new divine nature, which cannot sin because it is divine. When believers sin, it is just that old Adam has taken control for a moment, and it is through the old nature that we continue to sin. Les emphasizes that in God's eyes, old Adam is dead; that is why He can declare us justified, just as if we had never sinned, because old Adam is dead and gone so far as God is concerned.
For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.