When Jango Fett kills accomplice Zam Wesell with a saberdart in the first act of Senator Pe Amidala, provides rationale for the decision to terminate Wesell, but the choice is more than just another killing for the accomplished Mandalorian bounty hunter.
Prior to the events of Attack of the Clones, Fett recruits Wesell, a shapeshifting bounty hunter who had worked with Fett in the past, to assist him in assassinating the influential Pé. After Wesell’s first attempt on her life is foiled when his bomb blows up a decoy ship instead, Fett instructs Wesell to attack the Senator again, this time with deadly kouhun arthropods. Jedi Anakin Skywalker and Obi-Wan Kenobi intervene to save Pé's life before chasing Wesell through the Coruscant cityscape, eventually apprehending him inside a nightclub. Before she can reveal her employer, Fett kills Wesell with a single long-range shot before blasting off into the Coruscant sky.
According to the Star Wars Legends novel Boba Fett: Fight to Survive, Fett decided to terminate Wesell to prevent her betrayal. In the novel, Fett tells his son Boba that Wesell was about to expose him as her employer and that her death was an inevitable consequence of her betrayal, even going as far to say that “she would have done the same if it were me." Given that Fett chose Wesell to help in a mission that he could have done himself — he is even the one who provides Wesell the kouhuns — it is likely that Fett, the template of the clone army, wanted to use Wesell as a buffer to protect himself in the case that the mission went awry. When the Jedi captured Wesell, Fett’s decision proved to be a valuable one, allowing him to escape the Jedi’s grasp with a Kaminoan saberdart as the only incriminating evidence.
However, the killing took a toll on Fett. The age reveals that the elder Fett was shaken by Wesell’s death, even if he believed it to be a necessity. His face is described as “even sadder and older than usual” when he prepares to tell the news of Wesell’s death to his son, and he hesitates to reveal himself as her killer. Fett’s crestfallen appearance and despondent tone in telling Boba about Wesell’s death runs contrary to his typically stoic demeanor and his immersion in the bounty hunter lifestyle, but makes sense given their history working together and her implied relationship with Boba.
Jango Fett’s painful decision to terminate his partner in crime provides a rare glimpse under the iconic Mandalorian helmet, offering insight into his life as a bounty hunter and humanizing one of the galaxy’s most feared killers. The moment also underscores the devastation that Boba Fett feels, the first of Fett’s multiple encounters with death and loss in Star Wars: Episode II - Attack of the Clones.
