I can't take an answer to that question that places it on God. I sketched something during the discussion, and this is it:
Jesus had to die because we had to kill him.
The bastard was right, and we just won't stand for it.
We don't want God's justice; we want our judgment -- and our judgment killed Jesus.
Can protithemi -- "God set him forth as an hilasterion" -- be taken as God's guilt in Jesus' death? I think not. I refuse to translate hilasterion as either "propitiation" or "expiation" because both are non-sensical in the Hellenistic context. They set up a false dilemma in which God must soothe Godself by the death of God's son. I refuse to believe it because it is absurd. Since the context of Romans is Judean, but Pharisaical, I also see no reason to translate it as "mercy seat" in terms of Temple reference. The Pharisaical adaptation of Temple piety to daily life may have some metaphorical meaning for that component of the Holy of Holies, but we don't have it. What is an hilasterion? It is a sign of graciousness, from above, or an offering in return for graciousness, from below. Humans provide them as votives. But God provides this one, as that for which we might set up votive offerings of thanks. And what is God doing in 3:21-26, if we lift our eyes from this word and the mistaken blood-equals-slaughter-of-sacrifice association?
But it is now the case that, aside from Torah, the justice of God is manifest, being attested by Torah and the prophets: the justice of God through the trust of Jesus Christ, coming to all the trusting ones. For there is no distinguishing mark; indeed, all have sinned and are lacking the glory of God. All are being gratuitously justified by God’s grace through the redemption that is in Christ Jesus. God set him forth as a sign of graciousness through trust in his blood, to indicate God’s justice by the releasing of sins that had already happened, in the self-restraint of God. God set him forth as a sign of graciousness, toward the indication of God’s justice in the present time -- in order to be just, and justifying those who partake of the trust of Jesus.
And this, after 2:1-3:20, in which Paul establishes God's judgment as perfect, equitable, and inescapable upon every living creature, Judean, god-fearing Gentile, and godless Gentile all alike. This is Paul's utter antithesis to the provocation issued in 1:18-32, to partiality that excludes on the basis of human judgment, even human judgment that presumes to use Torah and know the mind of God. We are atoned, if that is the right word, not through God's judgment -- which is far superior to ours in every sense in which we might call for perfect judgment -- but through God's justice, God's tzedakah. Yes, that's the word -- God's righteous gift, given with no intention of entangling in debt, enabling the recipient to stand, as Rambam says in the Mishnah. In Christ we are all named Zedekiah, Tzidki-yahu, God is our Righteousness.
But I haven't answered our question there, have I? Why did Jesus have to die? I still believe my answer, hastily sketched in class. God put him forward, into a situation in which he would likely die, into a situation that would react violently against God's justice shown in him, even knowing that he would die, because God is righteous, and gracious, and eternally holds out hope that it will work differently this time. Because God can afford to lose to our sin, every time, and not actually lose anything. God will always choose to play the game with us, and God will always win, because the result will increase the good for us no matter how much bad there is. (This is because God never plays the game against us.) And so God did the thing right, and Jesus did the thing right, and we who were there at the time did the thing both right and wrong, some in every direction, and Jesus died, because we turned him in, and we stood by, and we schemed to free him in ways that were probably never going to work anyways, and we killed him, and we mourned him, and we did a million other things. And yet God still did the thing right, because death and sin did not win. God raised Jesus, and demonstrated that he is the son of God, and still provided the leverage by which we are freed and saved from our sin, even when after the fact we still did a million other things in every direction in response.
How's that for an atonement theology?