Dark Knight returns is meant to be seen as a kind of opera. It starts with Batman as an old Adam West version of Batman, with grey and blue in his costume. As the comic goes on the costume becomes armor and the world gets more violent, and by the end of the book he's riding a horse fighting Superman in a nuclear winter. Epic, operatic. Start small and sad, end huge and sad.
Conservatives tend to argue it was grounded and realistic/anti-liberal which I find very cute.
Reagan is portrayed as someone willing to throw the world into nuclear winter with a goofy smile on his face. For basically nothing, casually.
Comic books have been written by guys on drugs and anti-fasicst propagandists since the 1930s so of course they're going to be, more or less, left leaning in rhetoric. But the primacy of violence as a solution to all problems is explicitly fascist and is a major theme in a lot of comic books. So there's room for debate about what they have "always been"...