Under Capricorn

In 1831, Sydney is a frontier town, full of rough ex-convicts from the British Isles. The new Governor, Sir Richard (Cecil Parker), arrives with his charming and cheery but indolent nephew, the Honorable Charles Adare (Michael Wilding).
Charles, who is hoping to make his fortune, is befriended by gruff Samson Flusky (Joseph Cotten), a prosperous businessman who was previously a transported convict, apparently a murderer. Sam says that because he has bought the legal limit of land, he wants Charles to buy land and then sell it to him for a profit so that Sam can accumulate more frontier territory. Though the Governor orders him not to go, Charles is invited to dinner at Sam's house.
Charles discovers that he already knows Sam's wife, Lady Henrietta (Ingrid Bergman), an aristocrat who was a good friend of Charles' sister when they were all children in Ireland. Lady Henrietta is now a alcoholic who is socially shunned.
Sam invites Charles to stay at his house, hoping it will cheer up his wife, who is on the verge of madness. The housekeeper, Milly (Margaret Leighton), has completely taken over the running the household, and is the one who secretly feeds Lady Henrietta alcohol, hoping to destroy her and win Sam's affections.
Gradually, Charles restores Henrietta's self-confidence. They become closer and closer, and eventually they share a passionate kiss. But Henrietta explains that she and Sam are bound together most profoundly: when she was young, Sam was the handsome stable boy. Overcome with desire, they ran away and married at Gretna Green. Henrietta's brother, furious that aristocratic Henrietta had paired up with a lowly servant, confronted them. Her brother shot at them and missed; she then shot her brother fatally. Sam made a false confession to save her, and was sent to the penal colony in Australia. She followed him and waited seven years in abject poverty for his release.
After listening to Milly's greatly exaggerated stories of what Charles did in Lady Henrietta's bedroom, Sam becomes furious and orders Charles to leave. Taking Sam's favorite mare in the dark, Charles has a fall and the horse breaks a leg. Sam has to shoot her dead and, in a subsequent struggle over the gun, seriously wounds Charles. Sam will now be prosecuted again for attempted murder. At the hospital, Henrietta confesses to the Governor that Sam was wrongly accused of the first crime of murder; she was the one who shot and killed her brother. By law she should be deported back to Ireland to stand trial.
Milly, still plying Henrietta with drink, is using a real shrunken head to fake hallucinations. Milly then attempts to kill Henrietta with an overdose of sedatives; she is caught in the act and ordered out in disgrace.
The Governor, Sir Richard, has Sam arrested and charged with the attempted murder of Charles. Sir Richard ignores Henrietta's claim that Sam is innocent of both crimes. However, Charles decides to bend the truth; he says, on his word as a gentleman, that there was no confrontation, and no struggle over the gun. It was all an accident.
Finally we see Sam and Henrietta together smiling at the dock. They bid Charles a fond and grateful farewell; he is going back to Ireland.

Question: Who took Sam's mare in the dark?
Answer: Charles