Bonhoeffer's point is precisely that we can often be held back by these things, which are not necessarily gifts of grace. For example, virtue is acquired through habitus, that is, through effort (excepting the theological virtues of faith, hope, and love), conscience is a developed faculty, etc. What brings us to perfection is love, without which "I am a noisy gong or a clanging cymbal" (1 Cor. 13:1). We love because we are loved and God's love is Christ. So, He becomes that final standard, the criterion, by which we judge everything. I think we can see Christ crucified as the perfect example of what Bonhoeffer is writing about, which is the sole criterion by which he judged, a judgment that caused him to act in such a way that he, too, was executed.
Too often we reduce things to our measure. Note that Bonhoeffer writes: "his" before reason, principles, conscience, and freedom. This "his" transfers to "mine." Shaky ground, indeed! I am quite certain that if he had remained content to judge his circumstances and what was required of him by his own measure, Bonhoeffer surely would've taken a different course of action. Instead, in imitation of the Lord, he gave all of himself, which I am sure now seems to him as not a huge sacrifice at all.