In order for Christ to legally qualify to be our substitute and representative
There is a problem here. The language of the gospel is to convey various spiritual realities but they may not be quite in the way you suppose.
Imagine you smash a window of a house. Now the owner of the house could ask you to pay for the repair.
What if the reason you smashed the window because you were scared of the owner, and thought all the things that went wrong was because the owner refused to make things easy for you. Just replacing the window free of charge would make no difference. You would still feel the owner was bad, and everything was unfair.
So instead of replacing the window, the owner came and lived among us, was friendly, open, listening, healing, everything you would expect a loving perfect human being could be. But He was rejected because he was different and keeping things exactly as they are was what mattered to us. So we killed Him. Because the problem is with us, not Him. He is innocent.
It is only through this sacrifice could we see the reality. We are the sinners, he is the one who sees and is perfect, and is able to save. By communing with Him that we find life and reality.
He became man not to fulfill some odd equation of justice, but to show His nature does overcome everything, sin itself.
It is not innate within humans to sin, it is without communion with the Lord that we get the wrong perspective, things spin out of control and we sin, feeling justified in our deeds, not realising how lost we really are.
Now God saw the only way to reach lost sinners was to create this concept of an equivalence of a sacrifice that would cancel out the debt. The debt was rebellion, lostness, being alone, and the life is communion, love and acceptance.
If you get hung up on this concept, it is because you are probably still on the outside and have not seen the reality of how love works.