Starr articulates several reasons for keeping her role as the witness to Khalil’s murder a secret. In chapter 9, she express her dilemma in a series of rhetorical questions:
What if somebody knows I’m the witness? What if they know that it’s my fault that cop hasn’t been arrested? What if we come across some cops and they know who I am?
One reason could have to do with how Starr blames herself for the events that followed the shooting. She seems to think that it’s her fault that the police officer who shot Khalil hasn’t been put in jail. If she’d been a better witness or provided more compelling testimony, then, in the mind of Starr, maybe the police officer would’ve had to face consequences.
Additionally, Starr is worried that if word leaks that she’s the witness, other police officers will target her. She’s fearful that the police will do something bad to her or her family in order to prevent her from possibly talking further.
Near the end of chapter 9, Starr lists another reason for keeping her witness role a secret. This has to do with her desire to be “normal.” She doesn’t want to be the person who had to witness her friend get killed. She wishes that she could stick to the kind of conventional teen life that she feels she possesses when she’s around her white boyfriend, Chris.
As for why Starr finally decides to use her voice, think about how Kenya, as well as her dad’s own violent interaction with the cops, motivates her to speak out publicly.