Here's an excerpt to Andrew Zirschky's thoughtful article concerning Bonhoeffer and bin Laden (I'm proud to call this guy "friend"--Andrew, that is, though being friends with Bonhoeffer would have been pretty cool):
In Ethics, Bonhoeffer argued for extraordinary situations in which none of the options available for action or inaction could avoid guilt, unrighteousness and sin. On such rare occasions, says Bonhoeffer, we may find ourselves in a place where we must not "decide simply between right and wrong and between good and evil, but between right and right and between wrong and wrong."  In those times, both action and inaction lead to evil. The tragedy of our world is the evil into which we are drawn, even when we hope to remain aloof. This is why we as Christians cry for divine salvation. Human action is not enough to combat the evil that persists in our world and in our own hearts. As one commentator on Bonhoeffer has said, "Tyrannicide is sinful even if it is the least sinful option remaining." 
Check out the rest here.
 Mark S. Brocker, "Bonhoeffer's Appeal for Ethical Humility" online athttp://www.elca.org/What-We-Believe/Social-Issues/Journal-of-Lutheran-Ethics/Issues/August-2003/Bonhoeffers-Appeal-for-Ethical-Humility.aspx
 Dietrich Bonhoeffer, Ethics (Fortress Press, 2005), 245.
 Eric Meyer, "Violence and Disciples: Bonhoeffer on Resistance and Responsibility," Regent College, unpublished paper, April 10, 2007. Accessed at: http://ericdarylmeyer.files.wordpress.com/2007/04/final-paper.doc