The substitute was the animal, but that was insufficient which is why God had to come himself to sacrifice himself once for all. This is all in Hebrews.
The animal sacrificed wasn't a substitute, but was inferior because once it was slain, it remained dead,
So Christ was onceoffered to bear the sins of many;and unto them that look for himshall he appear the second timewithout sin unto salvation. Heb.9:28
Of course not, there still needs to be justice, but that justice is fulfilled only on the cross.
Justice isn't fulfilled on the cross. Justice comes later,
Dearly beloved, avenge not yourselves, but
rather give place unto wrath: for it is written,Vengeance
is mine; I will repay, saith the Lord. Rom.12:19
What does this have to do with Christ on the cross?
This,
Ye have heard that it hath been said, An eye for an eye, and a tooth for a tooth: But I say unto you, That ye resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also.....Love your enemies.....That ye may be the children of your Father which is in heaven
Mt.5:38-39,44-45
God will forgive those who repent of the sins committed against him, even the murder of his Son. Eve for eye will take place at his judgement seat,
Do not think that I will accuse you to the Father: there is
one that accuseth you,
even Moses, in whom ye trust. Jn.5:45
For as many as have sinned without law shall also perish without law:
and as many as have sinned in the law shall be judged by the law; Rom.2:12