Final answer:
The brutality of the murder is illustrated by the close-quarters use of bayonets and muskets, a detailed account of a young boy's violent death, and the inhumane burning of the bodies.
Step-by-step explanation:
Three details that demonstrate the brutality of the murder are:
The use of bayonets and musket blunts suggests a very personal and violent form of killing, as these weapons require close proximity to the victims and considerable force to be lethal.
A youth not seventeen years old being seized by his assassin and the graphic description of him being 'all torn and cover'd with the boy's blood' paints a vivid and horrific picture of the murder, highlighting its savagery.
The act of burning the bodies at 'eleven o'clock,' implies not only the murderers' desire to dispose of evidence but also their disregard for the humanity of their victims, adding to the cruelty of the events.