I agree with this, but I still think she takes too damn long to get there. I mean, just how many times does a person have to prove to you that they're a person before you believe them? I lost count of how many times it took for Clare to believe Billy.
Thomas Jefferson went his whole lifetime with people like Benjamin Banneker writing him letters and pleading/arguing that Black people should not be property and he was never swayed. I find it believable that it would take Clare two days to get it through her head that this person she'd stereotyped had the same kind of human experiences as her.
In fact, I think that a big part of her journey as a character is realizing the enormity of what is happening to Billy and his people. That if someone even lays eyes on him, they might kill him just because. That his family has been destroyed not because of one sociopath sadist, but because an entire country has made it their business to wipe them out. I think that in her shock and self-centered drive she can't see it at first, but she gets there. And at first she is so revenge-focused that she doesn't see him as a person, but more a means to an end.
I get what you're saying, but it's not like Clare couldn't do what she'd come to do. She proved that when she caught the first guy (the one who, frustratingly, deserved it the least). It also irked me that she dragged Billy into this mess and then he was the one to clean it up and pay the price for it all. All of that is believable enough, and I like realism to a point, but if I'm going to root for a protagonist like Clare I really need more balance than what The Nightingale offers.
I think that part of why she doesn't kill the men is that she's realizing what will happen to Billy if he's complicit in the murders, something the man from the couple at the end makes really explicit: "without you he won't survive". She begs him to leave, and it's something that I think deliberately parallels the way that she begged her husband not to confront Hawkins. She's already had one good man stand up for her and pay with his life, and she doesn't want the same thing for Billy. It would have been one thing out in the wild, but doing it in town is a different story.
I also think that Billy isn't just some bystander. He notes that Hawkins is "the worst one", and he's a potent representative of the army that has destroyed all of Billy's people and family. The army (and maybe even specifically Hawkins) have made Billy help them find indigenous people to kill and that weighs heavily on Billy's mind.
Like, I also wish that we'd seen Clare cut Hawkins throat ear to ear. But I think that Billy taking the final revenge--and reclaiming the indigenous ways that were stolen from him as a child--was satisfying.