Why was she murdered?

At the end of the first season of Carnival Row, we find out the true motive for the killings:

to find the illegitimate son of Chancellor Breakspear and kill him.

With that in mind, these murders and their order make perfect sense:

1. Aisling, because she's the mother of the said son and because the fake blackmail letter made her the first link towards her long lost son.
2. Finch, the headmaster of Philo's former orphanage, was the next target in that pursuit, because Aisling didn't know what became of her son, but she did know where he was left. The quest would've been over here, as he knew who the boy is, but his liver never made it to the murder, and she never got to learn his secrets.
3. Dr. Morange was another hint that came from Aisling, as he is the one who cut off the wings of her half-blood soon. It was quite obvious that he didn't know the secret either (he never showed Philo that he knew), so that was a dead end too.
5. Chancellor Breakspear('s liver) finally provided the answer.

What I don't understand is why was it necessary to kill

4. Aoife (Haruspex)?

She is the closest thing to a friend that the murderer has, she didn't know anything really important at that point, she said she'd learn who the culprit is just before her death,... so, why kill her? It feels that, without her death, the murderer would've probably succeeded, as Aoife is the one who told Philo who to look for and that she has Vignette.

What am I missing?

plot-explanation carnival-row

Best Answer

Spoilers, obviously:

First, Aiofe went to Piety right after having her vision, revealing that she learned the identity of the person controlling the darkasher moments before the darkasher killed her. Piety had one motive to protect her secret.

Second, Aiofe did not believe that Philo was the man who committed the murders and would eventually learn more about Piety's secrets. It would also be in Piety's best interest to keep a haruspexing witch's eye away from learning more about what's going on.

Aiofe would have learned that Philo wasn't the one to create the darkasher (since the murdering one was made well before Aiofe's one for Philo), and looked deeper into the true cause, discovering that Piety stole the secrets of her magic.

Third, Aiofe knew the truth that Longerbane was not the one to capture Jonah. This in and of itself wouldn't have caused her death, but in addition to knowing other things tipped her out of Piety's favor.

Likely, Piety thought she knew too much about her ambitions to keep alive.