Of all the villains that the Z Fighters face in Dragon Ball Super, few are as twisted as the evil Zamasu. An apprentice Supreme Kai from Universe 10, Zamasu grew to resent the mortals across the multiverse and plotted to eradicate them from existence while ascending to ultimate power. And while Zamasu was eventually thwarted as he launched his plot in Future Trunks' timeline, the evil god may have succeeded if he had properly used the Super Dragon Balls to carry out Project Zero Mortals, his aptly named master plan.
Seeking a stronger physical form to advance his plot, Zamasu uses the Time Ring he acquired from Universe 10's Supreme Kai Gowasu, whom Zamasu murdered, to travel to yet another divergent timeline in Universe 7. Singling out Goku as one of the mightiest warriors in the entire multiverse, Zamasu gathered the Super Dragon Balls, the most powerful Dragon Balls scattered across the multiverse itself, and wished them to exchange bodies with the Goku of this timeline. Dubbing himself Goku Black, Zamasu channeled his own divine powers into Goku's impressive Saiyan abilities to become the ultimate villain and attacked Future Trunks' time, where another version of Zamasu existed but had not yet carried out his fiendish plot. Teaming up with this counterpart, the two Zamasus used the Super Dragon Balls to make their composite body immortal.
With Zamasu possessing the most potent Dragon Balls in existence, it certainly begs the question: why didn't he just use the Super Dragon Balls to wish all mortals out of existence? It's a reasonable question, but the Eternal Dragons of other Dragon Ball sets have previously displayed limitations on the types of wishes they can grant. During Dragon Ball Z's Saiyan Saga, the Z Fighters attempted to use the Dragon Balls to eliminate the approaching Vegeta and Nappa, only to discover that this was impossible as both Saiyans were stronger than Kami, the creator of Earth's Dragon Balls.
While it stands to reason that the Super Dragon Balls, capable of restoring the entire multiverse after it was erased by Grand Zeno, should be able to accomplish a wish that would wipe out all mortals, this may be beyond the limitations given to the Eternal Dragon Super Shenron. Created by the Dragon God Zalama eons ago, the Super Dragon Balls are described as having no limitations on wishes. However, it's possible that Zalama may have created safeguards to prevent them from being used maliciously, as Zamasu would have if he'd used them to complete Project Zero Mortals.
Zamasu is one the most cunning, devious figures introduced in the entire Dragon Ball franchise, outmaneuvering his opponents and staying one step ahead of them with the Super Dragon Balls. However, if Zamasu was able to use the Super Dragon Balls to execute his master plan, he could have defeated the Z Fighters without lifting a single finger himself. While this could be due to unexplained safeguards placed on the Super Dragon Balls, it is just as likely that Zamasu may have preferred to do the dirty work of wiping out all mortals himself, with the villain already seen relishing the opportunity to murder Goku's family after taking control of his body.
