Dent never turned "evil". After the transformation into Two-Face, he sought justice just like he did as Harvey, but in a different manner. In his quest, he spared the Joker, killed one crooked cop, spared the other, killed Falcone's driver, and then tried to kill Batman, himself, and Gordon. Remember, he left everything to chance after becoming Two Face by flipping the coin. That was the only way for true justice in his mind.
He went after everyone who was involved in Rachel's death. Like Batman, he became a dark knight, a vigilante who sought justice against those who in his mind had done wrong. He just saw justice differently than the caped crusader (aka a bullet in the perp's head). I liked how Nolan took the "one bad day" theme from The Killing Joke and applied it to Harvey.
All in all a fantastic movie, the best of its kind. I give it two thumbs (and 8 fingers) up. 
