Final Crisis by Grant Morrison and J.G. Jones remains one of the most divisive of the various Crisis events in DC's long history. Part of it Morrison's trademark deep dive into mysticism and comic book lore alike, but another part is surely some interesting choices in the narrative - like how Batman literally pulled the trigger on Darkseid.
While Batman has famously has refused to kill even the worst of his rogue's gallery - including homicidal psychopaths like the Joker and Victor Zsasz - Darkseid is no ordinary criminal. By issue six of Final Crisis, Darkseid has unleashed the Anti-Life Equation on the Earth, reducing it to a near-post-apocalyptic state, with dozens of heroes and villains enslaved as his Justifiers. To counter Darkseid, Batman brandishes a gun (breaking his other major rule) which is loaded with the exact bullet Darkseid used to kill Orion. The two fire on each other, with Darkseid struck in the shoulder and Batman apparently flash-fried by Darkseid's Omega Beams. Wounded, Darkseid tries switching bodies to Detective Dan Turpin, but the combined speed of Wally West and Barry Allen lead the Black Racer straight to Darkseid, who is removed from Turpin's body and dragged to the afterlife. And if that weren't enough, the not-quite-dead Batman reappears in prehistoric times in the story's stinger.
To be fair, this isn't the first time Batman has killed, or even used firearms. The Batman of the Golden Age, following his first appearance in Detective Comics #27 in 1939, was heavily inspired by pulp characters such as The Shadow and regularly brandished a gun. However, in the fifties, the Batman comics often moved further away from the pulp roots, often taking on more goofy and colorful antagonists, leading to the extremely popular Adam West series in the mid-sixties. When Batman returned to his darker aesthetic in the seventies, thanks in no part to the efforts of Denny O'Neil and Neal Adams, a conscious decision was made for Batman to no longer kill or use firearms.
While Final Crisis remains a controversial comic for a variety of reasons, it does demonstrate how far Batman needs to be pushed to break his most important rule. Interestingly, Batman only breaks his rule on guns when there's no other way to protect his friends, and only chooses to do so at the moment of death (or more specifically, getting thrown back in time some sixty-five million years). This reinforces Batman's character, as it shows Batman doesn't break his rule lightly or look for a loophole, but instead, chooses to do so knowing it will quite literally be the death of him.