Manuscript number eight included in the Works edition of Bonhoeffer's Ethics is actually a re-working of manuscript seven - given the title History and Good (2). The themes are very similar, but I can see why the editors have elected to include both versions. Bonhoeffer approaches the same questions from different perspectives, and whilst the second version is certainly tighter and clearer, the first is worth a read to see how the thought develops. Given that it is essentially a repeat, though, here are a few thoughts with less quotes and more of my own interpretation.
In comparison to the first version. this second version puts Christology much more front and centre: it is because of Jesus Christ that we find ourselves in positions of responsibility towards one another. Because the God who has become man is our neighbour, making us neighbours of God and one another, we are placed in relationship with God and our fellow man. What I find fascinating and helpful about this is that, if I'm reading it right, Bonhoeffer makes Christ the source of our ethical responsibilities, the limit of those responsibilities, and the shaper of our responsible actions.
Christ is the source of our responsibilities because, as mentioned, it is he who brings us into relationship with God and one another. Christ is, in a way, the mediator of all our relationships - we see God and others through him. As such, he is the word of God which we hear, and to which we respond in all genuine responsible action (that is what makes it responsible). He is also the one in whom God and the world are bound together and reconciled, and therefore the only one who can make action in the world a genuinely responsible action - an action of significance in the sight of God.
Christ is the limit of our responsibilities because he has ultimately taken responsibility for the world. Therefore all our action takes place within the sphere of relativity; we do not deal with absolutes and ultimates. He is himself the only absolute and ultimate left to us. It is only because our responsibility is limited in this way that we can actually take any responsible action at all! Otherwise we would be frozen by the weight of it all, or we would construct an abstract ethical system in order to clarify our choices. As it is, knowing that ultimate responsibility is his, we can weight the situation and its likely consequences and make the necessary choice: we can act responsibly.
Christ is the shaper of our responsible actions, because in him we are called to vicarious representative action on behalf of all those for whom Christ has made us responsible. Jesus, in showing us what vicarious representative action looks like, has also shown us the way: it is the cross. That means both being willing to identify with the guilty and being willing to suffer for the other. Perhaps what it means most fundamentally is to trust God for justification whilst venturing the necessary action to which we are called, and which we must undertake without ever being able to certainly justify ourselves.
It's a powerful way of thinking, and I'm wrestling with what it means for us (me!) in the here and now to act responsibly, responding to Christ and answerable to him.