Answer: The murder of Scottish nobles in MacAndrew's barn
"But it's our wits that make us men".
The movie begins with a meeting in a barn. Edward the Longshanks has called a meeting of the Scottish nobles in an attempt to stop the fighting after the death of the king. Young William's father, Malcolm, is concerned because their neighbor, MacAndrew, was to come by after the meeting, and is going to check on him. In MacAndrew's barn, Malcolm, William, and his brother, John, find thirty Scottish nobles and their pages hanging from the rafters. Malcolm calls a meeting at his home to discuss retaliation with his neighbors against the English; he and John prepare to leave the next morning, and William wants to go with them, telling his father that he "can fight".
In reality, it is very difficult to know the truth about Wallace's early life. Some historians do believe, however, that he was born to an aristocratic family - not impoverished farmers - and that he was a knight long before the Battle of Stirling, as depicted in the movie. It is certain, however, that Scotland was ruled by Alexander III in 1276, at the beginning of the film, and that the English and Scots had been at peace for quite some time. Alexander III did not die until 1296; at that time of his death, there was a period of uncertainty as his young heir, Margaret, died on her way to claim the throne. Edward the Longshanks was asked to mediate as several made claims to the throne, and John Balliol was eventually chosen as the next king.