The seventh manuscript in the Works edition of Bonhoeffer's Ethics is the first draft of a section entitled History and Good. The section begins by making explicit that everything said so far only makes sense if we discard the common ethical framework of "an isolated individual who has available an absolute criterion by which to choose continually and exclusively between a clearly recognised good and a clearly recognised evil" (219). This framework must be discarded because there is no such isolated individual, there is no such choice, and there is no such criterion.
The discarded framework is an abstraction, and fails to recognise "the historicity of human existence" (220). An individual cannot be isolated from their historical situation and community. Rather, "a human being necessarily lives in encounter with other human beings", which leads to the individual having responsibilities towards those others (220). Note that these responsibilities are largely given, not chosen, and they provide the shape of our ethical lives. The norm for moral action becomes "not a universal principle, but the concrete neighbour, as given to me by God" (221).
With this historicity, we also lose the abstract recognition of good and evil, and are forced to recognise that rather than consistently choosing between good and evil (which are both known), each ethical decision is "risked in faith while being aware that good and evil are hidden in the concrete historical situation" (221). In other words, there is no clear ethical theory or principle which we can apply in a straightforward manner; to attempt it is mere abstraction, and can lead to the neglect of the actual responsibilities which God has given us. Wanting to be clear-cut, to always be right, can lead to ignoring the real situations which surround us.
In place of this abstraction, Bonhoeffer calls us to live in "accordance with reality" (222), always remembering that "the most fundamental reality is the reality of the God who became human" (223). We are called to think through the individual situations in the light of the event of reconciliation in Christ, and then to make free, and therefore risky, choices. These choices are made in faith - they "completely surrender to God both the judgement on this action and its consequences" (225). This is not acting blindly; choice is made in recognition of the seriousness of taking responsibility, a seriousness which is grounded in the fact that God in Christ has taken responsibility for us. But it is a recognition that we are not confronted by a black and white choice between the evil and the good, but by relative evil and good in complex situations. Only God knows all ends, and he has already taken responsibility in an ultimate sense; we are therefore freed to take genuine responsibility in a penultimate sense.
In the end, "the commandments of God's righteousness are fulfilled in vicarious representative action, which means in concrete, responsible action of love for all human beings" (232). What this makes clear is that the commandments of God are ultimately fulfilled by Christ, and that we take our part in their fulfilment only by conformity to him. This may involve, as it did for him, taking on guilt - although obviously not in the same way.
This is an ethic of being in the world, of being confronted by messy situations and unclear choices. But more fundamentally it is an ethic of being in Christ and shaped by him.