Complex sentence: Miss Gilchrist planted the idea that Richard Abernethie's death had been a murder-when Cora is murdered, it would seem that Richard's alleged murderer had killed Cora to silence her.
Split into simpler sentences: Miss Gilchrist planted the idea that Richard Abernethie's death had been a murder. When Cora is murdered, it would seem that Richard's alleged murderer had killed Cora to silence her.