I have read a lot about Bonhoeffer over the years and never cease to be amazed at how he is appropriated (and interpreted) in support of all sorts of theologies. Schlingensiepen's book is good, but occasionally veers onto the edges of hagiography, interpreting gaps in documentation with the most positive conjecture (“He must have had a conversation about…”).
However, what still stands out from any reading of Bonhoeffer is his lifelong insistence that there are no eternally valid ethical principles, and that Christians “in every historical situation [must] listen anew to God's commandments and … follow Christ.” (p.251) This enabled him to hold together two ethical stances that appear to be contradictory: (a) for the Resistance to kill Hitler whilst (b) rejecting euthanasia on the basis of the commandment 'Thou shalt not kill'. Schlingensiepen summarises: “Only a Christian who understands that he or she is free can make the right ethical decisions.”
It looks easy when put like that. But, the story of Bonhoeffer wrestling with the demands of commands (obedience) and decision (freedom) is a painful one to read. He didn't 'do his ethics' abstractly, sitting in a university library or a bishop's study (though he did study in studies and libraries…); he worked out his ethics on the ground, in the furnace of costly choice and agonising personal cost.
He was able to do this because of a fundamental vision: Jesus Christ stands before God (and Pontius Pilate) as “the obedient one and as the free one”, recognising that even the Gestapo can't bind the prisoner who is free to choose and who knows where power really lies.
I am now moving on to Tom Wright's Virtue Reborn for a more recent 'take' on ethics and character. But, the question that still haunts me about Bonhoeffer is how his theology might have developed further if he had not been executed at the age of 39. We round him off as if his theology was complete, but I would love to know how he would now be regarded if he had lived and developed and moved on.