Characters and their Tragic Flaws
Often times a protagonist's tragic flaw is directly related to the theme. Near the beginning of the Godfather, Michael Corelone tells his girlfriend Kay that he's not like his family. Of course, he ends up becoming the head of the family.
I take the central theme in Heat to be isolation/loneliness. It's set in a 100-mile wide city. In Mann's LA-follow-up, Collateral, Vincent opines about the disconnectedness of LA. So... cool setting. Pacino and De Niro's characters are workaholics, which isolates them. So... well done with their jobs. These elements are excellent vehicles to help to realize the theme, however...
McCauley repeatedly emphasizes walking away if the heat is around the corner, yet violates his own motto when he pursues revenge, which ends up getting him killed. He's not normally a vengeful guy. Van Zant tried to kill him, but he was able to put it aside for more important business. Is McCauley incapable of shedding criminal life?
Kilmer keeps his skin, but he's separated from his wife. He drives away. He doesn't really have a choice though. In a way, he's saved because she still loves him, but her minor character has more agency/arc than his more prominent character.
Pacino never gives up. McCauley's crew killed his brother-in-blue (Buffalo Bill), yet he pursues not because it's personal, but because it's his job. True to his word, he takes no satisfaction in gunning down McCauley, and holds his hand. Even though I've loved the movie for 25 years, this kind of false romanticism has bothered me in recent viewings.
It's tempting to call the whole thing homo-erotic. Typically in love stories, a couple must overcome near-impossible obstacles in order to authenicate and legitimize their love. In the case of Heat, the protagonists manage to bridge the cop-robber divide, so it's dramatically compelling.
Fans of the movie (like myself) are probably quick to call bullshit on most romantic comedies, but then here's a film that takes itself very seriously, and we're like, "Yeah, they hold hands at the end." Your fuckin' partner died earlier today. A bunch of other cops got shot.
Also, what does Pacino overcome? How does he grow from the experience? He found his soul-mate?