Being a member of the Justice League is one of the highest honors a hero can have, but that doesn't mean that every member is perfect. Some of the League's recruits have had far less heroic backgrounds than the likes of Superman. For example, Superman's nemesis Lex Luthor or the League's latest addition Black Adam. However, one member trumps both Lex Luthor and Black Adam for the title of worst Justice League recruit -- Atomica.
Rhonda Pineda first appeared as the New 52's version of the Atom in Aquaman #16 by Geoff Johns and Paul Pelletier. She helped defuse a bomb during the "Throne of Atlantis," drawing the attention of Cyborg. Cyborg invited Rhonda to the League along with other potential recruits, where she helped to put a stop to the robotic hero Platinum who had gone rogue in Justice League #18 by Geoff Johns and Jesús Saíz. These acts gained her a place in the Justice League, though Atomica's true intentions were well hidden.
The first time Atomica betrayed the League was when it was revealed she was working as a double agent for Amanda Waller's Justice League of America. Each member of Waller's American-based League was selected as a director counter to each member of the original League. Atomica's role was to take down Element Woman. Though betraying the Justice League is fairly bad, many well-known heroes were part of Waller's League. During a briefing, Atomica called herself a "bad person" for working with Waller. Her "guilt" was nothing but an act, but her words were more appropriate than anyone ever realized.
All the time she was working as a double agent for Waller's JLA, she was working against both teams for one that was infinitely worse -- the Crime Syndicate of Earth-3. While the League thought she was their ally the Atom, Atomica was working behind the scenes to bring down some of the League's heaviest hitters. She took a sliver of kryptonite from Batman's kryptonite ring and embedded it in Superman's brain, slowly poisoning him without anyone realizing. Atomica also used her hacking skills to bring certain subroutines in Cyborg's cybernetics to full sentience. The program not only stole League data for Atomica but it also took control of Cyborg's prosthesis and removed itself from its human host, becoming the murderous machine known as "Grid". Grid was instrumental in the Crime Syndicate's worldwide blackout that contributed to their reign of terror on Earth.
When Atomica revealed her treachery to the League, her true origins were told in Justice League #26 by Johns and Ivan Reis. While they had assumed she was just a shy girl from Ivy Town, she was in fact from Earth-3 just like the rest of the Crime Syndicate. Even before gaining her shrinking powers, Rhonda was pure evil. Along with her criminal and romantic partner Johnny Quick, she caused chaos across Earth-3. Together, they made Earth-3's Leonard Snart and Mick Rory fight to the death to decide which would live, then killed the winner anyway. All the while Atomica threatened their families, particularly their children. When both were dead, she wanted to visit Rory's family first since he had "more kids to play with."
During the Forever Evil event, Atomica and Johnny Quick ensured that there was no resistance to the Crime Syndicate's rule. They wiped out the Teen Titans by sending them through time. They then killed the Doom Patrol, with Atomica killing Scorch by bursting a blood vessel in his brain.
Atomica finally met her end at the hands of not one but two Luthors. In Forever Evil by Geoff Johns and David Finch, Atomica and Johnny Quick were taken down by Earth-3's Alexander Luthor, also known as Mazahs (Earth-3's Shazam). After killing Johnny Quick, Mazahs's powers leave Atomica permanently in a shrunken state. In the final issue, Lex Luthor spots her calling out for her dead lover. Luthor, knowing that the Crime Syndicate's reign started with her, crushes her under his boot.