Wait, this is a mystery? Really? How does that work? It seemed like straight-up, not very good historical fiction to me.
What is the mystery?
Anyway, getting beyond my bafflement at reading the name of the series, we get to my biggest issue with this one.
STOP. TELLING. ME. HOW. TO. FEEL.
I started exclaiming out loud every time the main character told me how I must be judging her, or must be feeling this way about her or must be thinking this thing. There were a lot of exclamations. I had to avoid reading this in public. It might have been embarrassing.
Look, main character-and-by-extension-author, you don't get to dictate how people react to your words. You just don't. And by constantly trying to predict them, you just irritate the fuck out of me. Mostly because you were almost always wrong. I was never the Miss Judgey-judgey, on-my-moral-high-horse you seemed to assume your readers would be, and thus had to apologize to about every 20 pages. Have your character be herself, with any of the complexity that you want to give her, and let your readers draw their own conclusions. They'll all be different anyway. But every time she apologized for her "darkness" and knew I was judging her, I was really judging Sara Poole. And not kindly.
Also, what darkness? She has an uncontrollable urge to kill, which she can totally control all the time, except the two times she was being attacked and killed in self-defense? Yeah, that's really uncontrollable. Yes, she's a poisoner, yadayadayada, but if you're going to have a main character who is a poisoner, fucking own it. And don't make this "darkness" that sets her apart from all other people in the world be expressed in killing in self-defense, and then, in the flush of adrenaline, not being sorry she did it. That's not sociopathy. That's...I don't know what it is, but it doesn't work.
And she's not a sociopath. From all the apologizing, you'd think so, but no. She is full of all the emotions. She is connected to and affectionate about all of the people. She just has idle thoughts of killing some of them. If that's what makes you dark and evil, well, most of people I know must be pretty damn dark.
Did I mention she's the Pope's Poisoner? Tell me that wouldn't have made a more entertaining title. Not a good title, per se, but funnier. She's the poisoner to Borgia. She's sleeping with Cesare. She's friends with Lucretia. They're all "dark" in the same way she's dark. They talk a good game, but they act in surprisingly predictable, not really all that bad, ways.
This is not a great book. But it wouldn't have gotten two stars if there hadn't been all that apologizing. You want to write about a female poisoner to the Pope, write about a female poisoner to the Pope. Stop saying you're sorry.