The Living and the Dead 2006 one on netflix

Lord Donald and Lady Nancy reside in the magnificent but run-down Longleigh House with James, their mentally disabled adult son. Nancy has fallen seriously ill and Donald is preparing to sell the house to raise enough money to pay for an operation. He arranges for the family nurse, Mary, to take care of Nancy while he leaves to tend to the sale. However, James wants to prove to his father that he can look after his mother on his own and decides to lock Mary out of the house. It isn’t long before James starts mixing his mother’s pills and forgetting to take his own medication, and as the stress of looking after his mother increases, so too does the severity of his own condition.