Events leading up to the murder-
When a young girl, Kaye had a fling with Cain, when his brother came back from the Crimean war. She fell pregnant to Cain, but was swept off her feet by Randolph and as he was more respectable, she married him. Cain took it hard, left the manor for several years, missed the wedding, then came back in disguise with a changed name.
He murdered Randolph and wooed Kaye during her grieving process and she changed her will to give him everything after finding out it was her beloved Cain from before.
Then the unresting ghost of Randalph started haunting the manor and Kaye held a seance to try to get rid of the ghost. Randolph uses the opportunity to possess one of the people there and shoot her in the bedroom with his old service Pattern 1853 Enfield rifle from the trophy room. He then ransacked the room to find the will and accidentally burnt the body he was in while trying to destroy it in the fireplace - the pain brought the possessee to their senses and they were the first to discover the body, but fled the room without telling anyone.
The ghost has since moved on, fulfilling its goals.
Everyone has a motive for murdering Kaye, except Cain, who murdered her husband and Gwynneth