There is a reason that Goeth is called Schindler's "dark half brother."  Keneally's point in constructing him in this manner is to show that there is a small demarcation between the nature of the human being moving from good to evil.  Goeth and Schindler share the similarities of similar upbringing and background.  They both love money and the share the desire to profit from what is happening around them.  They also are placed in the midst of the Holocaust, where there is not an overarching and clearly transcendent notion of what is "right" or "wrong."  In this, they both mirror one another because they are forced to make critical and essential choices as to what defines the path to take.  Their similarities make their choices all the more significant as it shows how human beings do have autonomy and freedom regardless of their condition and their situation.  I think that this becomes the critical element as to how both characters mirror one another and why it is so significant that they do so.  In this, the theme of human action and the capacity for redemption in a setting devoid of much of it becomes apparent.