One of DC Comics' darkest alternate realities, the brutal Flashpoint timeline, featured a war-ravaged Wonder Woman, with Diana truly becoming a villain after decapitating Aquaman's wife Mera, and wearing her crown as a trophy. While Wonder Woman is almost always portrayed as a true hero, her role in Flashpoint cements the fact that Diana's compassion and good heart are the result of nurture, and not nature, as she quickly becomes a villain after her mother and home are destroyed.
The Flashpoint timeline was created by a grief-stricken Barry Allen, using his Speed Force powers to run back in time to save his mother from the Reverse-Flash, but inadvertently creating an entirely new branched timeline by changing the past. This deranged new reality featured an emaciated Superman having been kept captive by the government since his arrival on Earth, Thomas Wayne as an uber-violent Batman after Martha and Bruce were killed, and Aquaman and Wonder Woman waging an endless war on each other. The relationship between Atlantis and Themyscira started out positive, with Diana and Orin (Aquaman's Atlantean name) getting married as a testament to the bond between the two nations, but their alliance quickly dissolved into all-out war.
Flashpoint - helmed by Geoff Johns and Andy Kubert - focused on an alternate Earth where everything was falling to pieces, but Wonder Woman's move towards villainy was one of the severe changes that a character went through in the massive crossover event. The marriage between Diana and Orin began to deteriorate after he began to see Mera as his concubine, and the dissolution was cemented when the Amazon Artemis, who wanted the marriage ended, killed Diana's mother Queen Hippolyta while wearing the armor of Orin's comrade Garth (Aqualad in main DC continuity.) After Diana brutally took over the United Kingdom, slaughtering countless humans after the destruction of Themyscira, a team of Atlanteans went after her, and Wonder Woman, with one swipe, viciously beheaded Mera and took her helmet as a trophy.
Queen Wonder Woman's darker, more violent tendencies won out, and she became nothing short of a major villain after killing Mera and sending Orin her corpse. At the same time, Emperor Aquaman was no hero in Flashpoint, sinking all of Western Europe and killing 60 million people to try and destroy the Amazons, but the last straw was when Aquaman saw Diana wearing the helmet of his slain wife in battle. She said it was meant to be a warning to her enemies, but it was clearly a petty trophy display meant to get back at Orin for his affair, and his nation's attempts on her life. Orin failed in his quest to slay Diana, instead activating an Atlantean weapon of mass destruction before Barry Allen was able to reset the timeline. Wonder Woman has been depicted as less-than-good in other alternate universe's throughout comics history, but she usually thinks she is doing the right thing, so her extreme turn into villainy comes as a true shock.
Decapitating her ex-husband's new wife and then wearing her helmet into battle against him was inconceivably cruel, and this mockery of a slain woman serves as a perfect reminder of how truly dark the Flashpoint timeline was. With Flashpoint's reality coming back to the DC Universe (for better or for worse), there is a chance that the comics will reintroduce Wonder Woman's evil alternate to the main continuity, and a fight between her and Prime Earth's Diana would be iconic. Many different realities have given Wonder Woman and Aquaman new looks, but only Flashpoint introduced a costume that pushed one Justice League mainstay to try and murder another.