I didn't like much of anything about the DARK KNIGHT sequel. It felt like Miller doing a bad parody of Miller. But you're right, it really went off the rails as the mini-series progressed.
If I remember correctly, the 9/11 attacks happened in the middle of it -- and Miller took a few months off, putting the project on hold halfway through, to recast it as an angry 9/11 response. This is how we get Metropolis getting nuked and a weepy Superman pawing through the rubble for survivors.