WARNING: The following contains spoilers for Batman: The Long Halloween, Part 2, now available digitally.
In both parts of the adaptation of Batman: The Long Halloween, it's hard not to feel sorry for Harvey Dent. He's really trying his best to heal Gotham City as lead district attorney, wanting to bring down mob bosses like Carmine Falcone and Sal Maroni. Sadly, his obsessions get macabre, so much so even the Bat thinks he might be the Holiday Killer. However, by the time Harvey totally loses it, mentally breaking down to become Two-Face, it's clear the real reason he became this villain isn't due to the man who threw acid on him -- it's because of the Dark Knight.
When Harvey and Batman first contemplated burning Carmine's money, the Caped Crusader offered him a coin to flip to make the choice. This encouraged Harvey to wonder if they could take the other path -- stealing the money, with no one knowing. Of course, Batman said no, but that began a spiral of Harvey leaving things up to chance. He kept the coin, flipping it the night Carmine tried to bomb him and Gilda, and since then, it's become his totem.
As Batman: The Long Halloween shows, the coin was given to a young Bruce the night Thomas Wayne stitched Carmine up to continue their uneasy business alliance, so Batman did feel like there was some energy to it that could heal the beast inside Harvey. But sadly, it just brings out the monster within the man, as Harvey keeps using it to justify or create avenues to bad moral and ethical decisions. He ends up realizing life is nothing but a duality: a yin-yang where only the strong will survive.
It's why, after Sal fakes a plea deal and throws acid on Harvey, thinking he's the one killing off Sal's people, Harvey loses himself fully and embraces Two-Face. He was hearing voices with Gilda earlier, experiencing a split personality as he started garnering clues that she was the real Holiday Killer with an old vendetta against Carmine's family. And because of this, plus the way he was pressured at work and how he kept pushing forward with his alliance with Jim Gordon and the Bat, Harvey broke. He grew desperate, and seeing the Bat hitting brick walls in his detective's chase made Harvey lose hope in the light.
Thus, after escaping the hospital in Batman: The Long Halloween, Part 2, he rounds up an army, starting with Grundy in the sewers. He then uses the coin in Arkham, freeing certain villains to help him attack Carmine's high-rise. And there, Batman's coin leaves its final, indelible mark as Two-Face admits he's lost the war within. He flips it and it makes him murder Carmine, leaving Bruce heartbroken.
Bruce saw this coming all along, which is why he kept trying to save his "friend." The Bat feels guilty because he helped warp Harvey's already-dented moral compass, driving him down this path to murder. Batman should have kept Harvey in the lines of justice, but instead, he was happy to let the lawyer bend, not break, the legal confines in their crusade.
It's why, when Two-Face eventually gives himself up to Jim and Batman at the rooftop Bat-signal, they feel like they've lost. The war has cost them Harvey's soul, and no matter how many mobsters are gone, to Bruce, this sacrifice is a price too high and not worth paying.
Batman: The Long Halloween, Part 2 is now available to watch digitally. It will also be out on Blu-ray on Aug. 10.