Canidia should've had better character development.
There is literally one moment in the whole series that explains her motivation – in the first episode, when Catilius asks her about her motives, and she replies, "Vengeance." Among her other horrible deeds, Canidia is poisoning the emperor while posing as his friend solely because she wants revenge. Revenge for what exactly?
Well, according to the (archived, currently absent) character description on the Mondo TV site, they were childhood friends and grew up together; they're about the same age as well. She was in love with Hadrian, but he chose another, and now he's oblivious to whatever evil Canidia is doing behind his back. There are many things he's oblivious to. The topic of her feelings/attitude towards Hadrian is never explored, which is, in my opinion, a pity. Gutta could be involved in their conflict as well, assuming he's a blood son of Canidia and not just a random corpse.
Canidia doesn't have to be a one-dimensional power-hungry religious fanatic. I'd love to know more about her backstory, relationships with the surrounding characters, her reasoning for being a High Priestess of a god who's highly unpopular in Rome. Come on, creators. I don't want your wicked old hag. Give me a spurned young woman, a dedicated practitioner, a pained mother, after all. Show people what, or who, made her into a monster.