The set-up: Maxwell Lord is influencing Superman to attack people. He’d already beat up Batman and now, he’s going after Wonder Woman, implanting the idea in Superman's head that she is actually Doomsday hunting Lois.
During the fight, Wonder Woman creates a diversion long enough to get to Lord, using the Lasso of Truth to get him to tell her how to stop Superman’s rampage. He says two words: “Kill me” and she does, shockingly snapping his neck. This becomes a huge deal when Brother Eye broadcasts the footage of her killing him to every television, which is arguably the more important rule being broken.
Wonder Woman is a tricky character to really quantify when it comes to her rules and morals. While she IS a moral person from a warrior race of women, her society is not necessarily opposed to killing, just murder. Writers do give her various morals, though you can generally tell which mindset the writers go in with if she’s carrying a sword or not.
The real rule broken here is a general rule of superheroics: the comfort rule. A hero, be they vigilante or deputized in some way, is on the side of the people and the greater good. They’re allowed to continue their actions with a positive outlook because they make the citizens of their respective worlds feel safe. The second that Wonder Woman snapped a guy’s neck in front of a camera, she broke the Golden Rule of Superheroes.
Learning to read from his parents' old comic book collection, ink has flowed through his blood since he was born! And now, the self-proclaimed Comic Archivist (at least on Youtube) brings his talents for comic book history and obscurity to WhatCulture! Be warned: he's a Cyclops fanboy and a Nightwing groupie!