how the murderer in the novel is identified

The investigator of the crime is Inspector Japp, who thinks that Alfred is the one who committed the murder. He refuses to provide an alibi, and he has the most to gain (They found an updated version of Emily's will, in which Alfred inherits the manor). Because he is so young, it seems like he is simply a man looking for easy money.

Alfred is found guilty of the murder, and he did it in coordination with Evelyn. Apparently, he had written a letter about killing Emily, and this is what made Emily so upset the day of the murder. Not being able to go another day without her telling someone, he decided to kill her that night.