During that period of history in Christian dominated regions, left-handedness was (quite literally at times) beaten out of children. It was believed that using the left hand was a sign of the devil. In the movie, Wallace was the child of a farmer--poor and uneducated. He may have been naturally left handed and it went "unchecked" because it wasn't really relevant or issue for concern. However, after his father's death, he went to live with his uncle--a man of obvious wealth. Wallace was given advanced (for the time) education and exposure to the upper class. It is entirely possible that he was taught/forced into using his right hand rather than his left at this point.
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