This whole poem leads up to the beginning of the Revolutionary War. The main part of "Paul Revere's Ride" isn't focused on violence and war, but images of battle and struggle and fighting are everywhere. We can feel the war hanging over the poem, filling it with excitement and tension but also sadness.
To give the sense of danger and excitement that makes the poem so dramatic, Longfellow makes the British soldiers more like an evil force than real people.
Although the poem is about the glory of the revolutionary spirit, Longfellow's descriptions of violence are meant to communicate the horror and sadness of war.