Less than half way through but to summarise...
The protagonist is in prison writing memoirs.
Has committed multiple murders but is inside for a murder they didn't commit.
Are murdering their relatives because their mother was wronged by wealthy relatives.
Has a close childhood friend of the opposite sex.
(And I have just guessed, and had it confirmed that it is the partner of the childhood friend whom the protagonist hasn't killed, but is in prison for).
Gets close to one victim by feigning enthusiasm for the victims slightly esoteric hobby.
Gets close to another victim by joining the family firm.
I really don't understand how an author can transparently plagiarise the plot of the well known classic film Kind Hearts and Coronets (or the original book Israel Rank perhaps), and receive great reviews.
Is changing the sex of the protagonist really enough to fool people into thinking that this novel is original?