"Though success can never justify an evil deed or the use of questionable means, it is not an ethically neutral thing. All the same it remains true that historical success creates the only basis for the continuance of life... - Bonhoeffer, Dietrich. Letters and Papers from Prison (p. 7). (Function). Kindle Edition.

It is an interesting thing success, as it tends to be a confirmation that one is headed in the right direction. But what of success in the endeavor of evil acts, what then do we consider? Should we consider that because they are experiencing success that then their acts are God ordained or the path is correct? It is possible that this is the case as the evil acts of one, is the judgment of another, and all that occurs is God ordained. This is probably why Bonhoeffer goes on to state:

"It is no good then behaving as an arm-chair critic and disputing the issue, for that is to refuse to face the facts... We must be determined not to be outraged critics or mere opportunists. We must take our full share of responsibility for the moulding of history, whether it be as victors or vanquished." - Bonhoeffer, Dietrich. Letters and Papers from Prison (p. 7). (Function). Kindle Edition.

In other words, it is a call to repentance for the vanquished in their part to bring the judgment of God upon themselves.