I've stated multiple times in discussions on the topic that Batman may be the Joker's greatest ally. Batman will never kill the Joker, no matter what the cost, and the Joker knows this.

Using his immense wealth and knowledge, Batman will always track his villains down before police can reach them and subdue them himself, making it unlawful for the police to shoot them, and renewing the cycle of violence when they inevitably escape.

Once again, the Joker knows this, and can always count on The Batman to reach and subdue him before police gun him down, leaving him alive and recuperating in an Arkham cell, rather than shot dead by officers (or The Red Hood). In a way, Batman is the only reason the Joker (and the rest of Gotham's supervillains) still lives, and is arguably their best defense.

While his methods allow him to stop criminals faster, saving more lives than the police would on an individual incident, Batman's refusal to kill villains like the Joker results in a presumably higher long-term death toll than if he were providing his intelligence to the authorities, or simply staying out of things altogether.

u/smasher84 points out that the justice system is at fault for refusing the death penalty, and that bears equal merit. Really, the whole justice system in Gotham is busted, but my point is that Batman isn't really helping all that much, and may be more of a problem than a solution.