The murderer??

Checked the spoiler warning just incase I’m right but what if it’s his sister? And not blondie?

First victim was let go because of poor teaching (as said by the lil sister).
Second victim(the Chef) nearly triggered blondie because of the steak order change, probably causing emotional distress at home, which the lil sis would know about.
Latest victim was rude to both blondie and FL, two people lil sis looks up too.
FL being killed in the novel as the fiancé could've been because of how desperately she pursued another man despite being engaged.

The murderer being a protective little sister makes sense as to why a lot of the victims link back to The Duke.

And the victims being killed via poison THEN being strangled to make it look like someone bigger and stronger did it? It’s definitely the sister.