Can you give some brief details of the murderer's character in The Murder of Roger Ackroyd?

Dr. James Sheppard is the novel’s narrator is a physician in the town of King’s Abbott. He serves as a companion-chronicler to Hercule Poirot, the brilliant detective who will eventually crack the very complicated case. Although Sheppard appears to be a genial, straightforward character, his complicity in the mystery will prove one of the great plot-twists of all time. Ultimately, Dr. Sheppard is revealed to be a shrewd, duplicitous, detached villain.