Warning! Spoilers ahead for Crime Syndicate #6!
In the final issue of Crime Syndicate, the death of Earth-3's Supergirl's is seemingly what caused DC's evil Justice League to become the unstoppable force that they are today. The entire motive behind the Crime Syndicate miniseries was to establish how these now-iconic malevolent versions of Earth-0's heroes became a team. Naturally, as issue six is the final chapter, the story has come to its climax. Everything that they endeavored to cover has been covered, on the villain side but also on Earth-3's heroic perspective.
The Earth-3 version of Supergirl — Ultragirl — was introduced on the final page of the series' fifth issue. Her return was not only a complete surprise to readers, but also to Ultraman (this world's Superman) who was completely unaware of her existence. As it's explained in this issue, when Kara Zor-El initially arrived on this planet she was so disgusted by her cousin's actions that she decided to keep to herself. But it seems like some other people knew of her existence, as suggested in this issue. It seems that she was a hero in the shadows, working with Alexander Luthor.
In Crime Syndicate #6 by Andy Schmidt, Kieran McKeown, Bryan Hitch, Dexter Vines, Steve Oliff and Alex Sinclair, Ultragirl dies at the hands of the Crime Syndicate, DC's evil Earth-3 Justice League. Specifically, she is killed by Atomica, the team's tiniest member who will be replacing Johnny Quick. The alternate Supergirl's death is suggested to have an enormous impact on the future of Earth-3. She had the ability to go toe-to-toe with Ultraman and even get the upper hand. Ultragirl's brute force could have evened the playing field against Earth-3's villains, if only she had better back up to deal with the Crime Syndicate.
Alexander Luthor, who it's indicated had a relationship with Ultragirl, suggests that Earth-3 stands no chance without her. And as it's shown in the timeline of events, Supergirl's death is the final blow to any hope for Earth-3's ragtag group of heroes. Unlike her cousin, Ultragirl was very similar to her Earth-0 counterpart given her heroic path. Ultimately, it's Ultragirl's death that leads to the Crime Syndicate becoming the malevolent rulers that they are today. If she were to have survived, it's possible that the Crime Syndicate could have been stopped.
Surprisingly, Ultraman is trying to keep Ultragirl alive in this issue, despite their fighting. This is because he's obsessed with the idea of having more Kryptonians around him. Even as the Crime Syndicate is officially formed, he is still annoyed with Atomica as he believes no one has the right to kill one of his own people. It's all part of his ego about being superior to everyone else. But if he had it his way and Ultragirl survived, it could have meant the end of Earth-3's evil Justice League. So, Atomica's decision to kill their version of Supergirl potentially saved Ultraman from ever being stopped, as the one hero powerful enough to put him down is now dead. Crime Syndicate #6 is available in comic book stores and on digital platforms now.