No other thread is necessary, observe:
Dark Knight’s approach tells a story of greys more reminisant of reality. Batman must break the law to uphold it. In an ironic twist, his zeal to clean the streets of gotham creates more pain and suffering in the world through the Joker. Another theme held over from the comics, is identity. Despite making the life of Batman much easier (and sparing the world from future suffering), batman cannot bring himself to kill the Joker, whereas the Joker cant bring himself to kill batman.
Now compare this to christian mythology
Following God (personified by Jesus) = good
Not following God (personified by Satan) = bad.
Deep indeed