Being exists in two different ways:
1) As absolutely good.
2) As relatively good.
Mode 1 is primary, and mode 2 is relative to the absolute goodness of mode 1. Notice that "evil" appears nowhere in the description of being. Evil is an absence of goodness, or in other words, an absence of being. To describe something as evil is to say that it is not as good as it might be. It follows that there is no such thing as "absolute evil" as there is "absolute good." Absolute evil is pure non-being; it is nothing at all. The only kind of evil is relative evil, and that evil is relative to absolute goodness rather than absolute evil. Relative good and relative evil really describe the same mode of being; the only difference is the degree to which being has fallen into non-being.
A battle of "good vs. evil" is therefore a contest of one of the following two types:
1. Absolute good vs. relative good.
2 Relative good vs. relative good.
Only in the case of a type 1 battle is the victory of good over evil assured. And only in a type 1 battle will the victory be one of pure goodness; i.e. the victory will be a victory of pure being over non-being. The primary example of a type 1 victory is, of course, the victory of Christ in His Death and Resurrection.
In a type 2 battle, the victory of good over evil is not assured; and even if the relatively better does win, the victory will be tainted by aspects of non-being, for only in absolute goodness is action free from non-being. Because a type 2 battle is always a mix of good and evil on both sides, there is always a temptation to despair that goodness can genuinely be advanced in such a battle. For we can always find in any type 2 victory elements of non-being (evil.)
To take an example, World War II is sometimes called "the Good War." If this is taken to mean that the better side won, then there is no problem with the term. If it is taken to mean that the better side won in an absolutely good manner, then the term is misleading; for no merely human enterprise can ever be free from evil, especially war. And the Allied victory in WW II had its morally problematic elements - the incineration of hundreds of thousands of German civilians in strategic bombing being one of them. Nonetheless, the Allied victory was both necessary and just in its final outcome, despite the morally problematic means it took to get there. The (relatively) good guys won.
There is a cast of mind that cannot except that the relatively good is just that - relatively good. When it discovers the evil hidden in any relative good, it despairs of the relative good altogether. It demands all or nothing. Thus, when some people discover the horrors of the firebombing of Hamburg or Dresden, their understanding of the war collapses into moral relativism; we are no better than the Nazis and the Allied victory was not a victory of good over evil.
In my original post on the Dark Knight, I said that the Joker is a "secret idealist." By that I meant that the Joker cannot accept the goodness of the relatively good. It's all or nothing for him. The Joker is the kind of person for whom the strategic bombing campaign obscures all moral differences between the opposing sides in WWII. This is a manifestation of the sin of pride.
Batman, on the other hand, understands that the relatively good is worth fighting for even if it is mixed with evil; that his campaign against evil is worth it, even if his own efforts are necessarily tainted by evil; and that the salvation of Gotham is worth it even if that salvation is effected by illusions. So, yes, we may point out the morally problematic nature of Batman's means to victory: He induces Commissioner Gordon to support a Platonic "noble lie." Is it a mark against his character?
That is hard to say. Batman is a "post-Christian" myth. That is to say, it exists in a universe in which the salvific and revelatory Incarnation, Death and Resurrection of Christ has not taken place; at least, no one seems to know about it. The Incarnation changed the meaning of the virtue of prudence by giving man an example of absolute goodness and also the sanctifying grace to perform it. Christ sets the standard by which man is judged in a Christian universe; absolute goodness is now the standard even if man can only approach it.
In a non-Christian universe, prudence does not have this relationship to absolute goodness. Prudence is the struggle to advance the good in a world where everything is tainted by evil. This is why the pagan Plato can advocate the use of the "Noble Lie", something no Christian needs to do, since the Christian is in possession of the Noble Truth of Christ. The standard of prudence in the pagan world is not absolute goodness; it is the effective advancement of the good in a world where good must compromise with evil.
So in the post-Christian world of Batman, the question about his character must be understood in terms of the meaning of prudence in that world. That Batman resorts to evil means is true; that such an act reveals a character flaw, I am not so sure. He did the best he could in the unredeemed world in which he lives.
The title of this post is the "Dark Knight as Mundane Christ". Like Christ, Batman sacrifices himself for the salvation of the world (Gotham). But Batman is merely a mortal man. He is not absolute goodness, nor does he have power over life and death. So his salvific act will necessarily include elements of evil (non-being), which Paul and Amos have been pointing out.
His victory is not absolute or permanent, but temporary and shaky. It is based on illusions the Gothamites have about themselves, Harvey Dent and Batman.
But it nonetheless remains true that Batman's victory is a victory of the relatively good over the relatively evil, and it fundamentally flows from a virtue he possesses and the Joker does not have, and cannot have: Humility. And goodness finally remains more powerful than evil, but not because the victory of Batman was inevitable. In any contest of the relatively evil vs. the relatively good, the relatively evil has the possibility of victory (as the Nazis came close to winning WWII.) But because it is based on being, the relatively good has more resources at its disposal, if it can only find the means to use them. In the case of Batman, he finds a way to use the good virtue of humility to defeat the Joker, a move the Joker cannot match because humility is unavailable to him.