What is frequently missing from discussions of casuistry is that the question of absolutes and objective morality really needs to be squared away in some sense for general principles to be even considered to apply. The other critical thing is, that unless you disbelieve in evolution, the brains which yield these moral judgements are constrained by the evolution of brains, which is in some sense critically dependent on those selfish genes for its expression and its meaning. In particular, there will likely be moral effects which express themselves in terms of the chaotic attractors of genetic systems, and which are even occasionally mere side effects of the logistics of reproduction (e.g., potential evolutionary psychological explanations for rape and monogamy). In many cases, these evolutionary effects will swamp any signal to be derived from rationally analyzing the situation and the principles involved.
The concentration camp scenario is likely one case in point. While the prima facie consequences are the difference between an extra person being killed or not, there may be deeper consequences as well. How productive as a species would it be for individuals to give in to bullies, assist murderers, and evaluate the result in terms of number of lives rather than quality and dignity. In particular, the human animal has evolved social behaviors that abdicating to the principles of expediency and an "all lives same" approach would throw into chaos (there are social systems which seem geared primarily for weeding out bad apples, regardless of the cost; what happens to such a system when people reduce their moral calculus to the level of a human abacus?). And the systems of reproduction depend upon the existence of values such as human dignity (a side of the coin of altruism) and fidelity. If humans start abandoning their love and commitment to sons, the consequences for reproduction are devastating. Genk and I tend to go round at times, as he engages in similar efforts to reason things out from first principles, and though he has an excellent analytical mind, he sometimes overlooks these broader effects, and how things interact at multiple levels. But then, i suppose we all do.
The problem is engaging in casuistry in the hope that you can reason from a bunch of specific cases to fewer general analytical principles. Ignoring the Kantian point that no size of heap of specific cases will ever equal a single generality, it misses the essential question of just what morality is at the bottom and what its purpose or purposes are. Without knowing those, your attention to detail and analysis will simply leave you looking at trees and never seeing the forest.