Yes, Batman did kill in The Dark Knight Returns comics in some instances, but it was within a specific context and for particular reasons related to the storyline.
One of the main themes is the idea of an aging hero. Batman is older but still feels the need to fight crime in a Gotham that has changed. Another theme is the blurring of the line between hero and vigilante. His extreme methods raise questions about justice. Also, the corruption in Gotham and society's reaction to Batman's return are important themes.
One key theme is the idea of an aging hero. Batman is older but still driven to fight crime in Gotham. Another is the moral ambiguity, as he often has to make tough choices that blur the line between right and wrong. Also, it explores the relationship between Batman and the city, how they are intertwined and how his actions affect the city's future.
Batman is the main character, of course. He's an older, more grizzled version of himself. Robin also plays a role. There's also Commissioner Gordon who has a long - standing relationship with Batman. And the Joker is a significant antagonist in the story.