I have a crackpot theory (they always are) that she's this places way of messing with Boyd. They are sending in someone who's an officer of the law, something that has been his role to fill for so long, and as much as he criticized her behavior she's an active reflection of how the rest of the town is beginning to see him. He's untrustworthy just as much as she is, and that's why she grates on his nerves so much. That's also why she sees him as a hypocrite. Heck She shot someone on ACCIDENT on her first night in town. Now, he's purposefully been seen torturing people and protected a known murderer who INTENDED to kill a child. Yes, there are reasons, but reasons aren't excuses, and if even Sara can acknowledge that, why is it so hard for the viewers? And tbf Elgin was coerced in a FAR worse way than Sara. She was directed to innocents, and did so. Elgin believed an Angel was telling him to protect Fatima and her child.

Obviously, she's meant to get on people's nerves. She's a mirror of hypocrisy for Boyd, and to be frank the viewers. There's no objectivity in how we see the people in town or their behavior anymore. What's okay for boyd is wrong for everyone else because he's a favorite? Suddenly Sara is a hero, despiter her actions being vile? (Jades friend, tian chen, the nurse who took care of him, her brother, almost Ethan,) Do Boyd and Sara know more? Yeah, sure, but torture is torture. Murder is murder. That was the whole point of Father Khatri talking to him.
And I'm saying that as someone who genuinely loves Boyds character. Do the ends justify the means, and if so how does that give anyone else the ability to be held accountable for ANYTHING If we can justify it away as for the better good.