Atonement is both penal and substitutionary – here is John Webster on what is happening on the cross:
He becomes, that is, the bearer of our sins. “Surely,” Isaiah tells us, “Surely he has borne our griefs and carried our sorrows” (53:4); and again: “the Lord has laid on him the iniquity of us all” (53:6); and again: “he bore the sin of many” (53:12). It’s easy to misunderstand this. If we’re not careful, we can think that what’s happening in the passion is that God is simply punishing an innocent victim for our wrongdoings—as if God simply requires that the punishment for our crimes should be enacted, and it doesn’t matter who is punished. But Jesus is not just a mute sacrificial animal. If he is like a lamb led to the slaughter, it’s not because God is victimizing him; it is because he is God himself fulfilling his own purpose; it is because he is God the Son, freely and lovingly acting out the will of the Father. “It was the will of the Lord to crush him” (53:10). That does not mean that God just vented his anger at sin on Jesus. It means that he, Jesus, the Son of God, is God himself bearing the wounds of our wickedness. God does not save us by sacrificing someone other than himself. God sacrifices himself. In his Son, God himself bears our sins. He makes himself an offering for sin (Hebrews 7:27). Or as Colossians puts it “in him”—Jesus—“all the fullness of God was pleased to dwell, and through him to reconcile to himself all things, whether on earth or in heaven, making peace by the blood of his cross” (1:19).
Webster, J. (2014). Confronted by Grace: Meditations of a Theologian.