> The choice is between trying stop an ongoing harm or not trying to stop it.
That is the normal choice that any person has to make, there's nothing Christian about this, nor about the decision that Bonhoeffer made. Any atheist would agree with him that you've got to minimize harm, etc.
The relevant choice for the Christian is to decide whether we're going to "use evil for good" in a way that only God can facilitate, by loving our enemies and dying for them, as Christ died for his enemies.
> What you present isn't a third option, it's just a particular mindset about the second one.
It's not "just" a mindset because it restricts (and expands) the options you can take. Isn't Bonhoeffer's stance "just" a mindset, because regardless of why you're killing your enemies, they end up dead anyway? An atheist killing Nazis and Bonhoeffer killing Nazis only differ in what they're saying, not really what they're doing. A Christian not resisting evil and loving their enemies is identifiable both by word and deed.
> Many christians do accept that as the only choice. Others finds that they are compelled to action, and consider this a personal failure to remain peaceful in the face of great evil.
You're construing this as a choice between action and inaction, so I don't think we understand each other yet. Was Jesus action-less when he came to the earth to die for his enemies? Or did he take action to not-resist evil, love and die for his enemies, and bear the sins of the world? What I'm suggesting as the Christian approach to this is very active, but not to kill our enemies the same way literally everyone else does. It's to feed our enemies when they're hungry, give them drink when they're thirsty, and stand in the way of the bullets when they're fired at others, all the while proclaiming our love for them, and God's love, so that they would quit trying to earn their righteousness by good works, and believe on Christ for righteousness. This is all and only what Jesus did, and bade us do.
> Not all sin is avoidable, a sinless choice is not guaranteed.
This might be your opinion, but I'm not sure you could find any support for this in scripture.
I'm also not sure that we can take Bonhoeffer's decision as anything other than the proposition that Jesus would have killed Nazis too. Is that the road we want to go down? (I presume he wouldn't be defending a choice to do something that Jesus wouldn't have done.)
Maybe the answer here is that I just need to read his book first.
> Contemporary Theology Understander: We cannot do evil so that good may come of it ... that is evil.
> Big Brother D, unable to escape the conclusion that he must kill nazis: Lord Jesus Christ have mercy on me, a sinner.
The problem with this is that Big Brother D is morally equivalent before God to the Nazi, so in deciding the fate of the Nazi, he's deciding his own fate: he's too evil to let live, he should be put down. This is Brother D acting like he's still under the Law, which says, "The man who does these [good] things shall live [and by implication, the man who does bad things shall die]." According to Jesus, God cannot and will not have mercy on a (n undeserving) man who does not have mercy on (undeserving) others. There is no forgiveness for those who do not forgive others. Bringing death on others is asking for death upon ourselves (and for consistency, everyone else).
Bonhoeffer can't have mercy for himself and death for others.
And isn't this the thing that sets Christianity apart? Otherwise Jesus' earth-shattering teachings become just another "treat people nice who are nice to you, punish the bad guys" idea like every other one out there. My personal feeling is that trying to give up this nigh-impossible teaching is trying to give up the heart of Christianity, the core beauty and truth. Christianity has nothing attractive about it if we rip out the core and everything else that depends on it, imo.
Sorry for the long post.