Haun's Mill was a tragedy. It is called a massacre, because it took place in a time of truce. "Because the attack was unprovoked in a time of truce, had no specific
authorization, and was made by a vastly superior force with unusual
brutality, it has come to be known as 'The Haun's Mill Massacre.' (Blair, Alma R., Encyclopedia of Mormonism) Haun's Mill was a small isolated Mormon community. That the attack was possible was a result of the people not heading the prophet's instruction of the isolated communities gathering to Far West. This request was not passed on to the Saints at Haun's Mill as presented in this discourse of Henry B. Eyring.
http://www.lds.org/media-library/video/doctrine-and-covenants/section-120-church-history?lang=eng
Blair explains the events of the day:
Thirty to forty LDS families were at Haun's Mill when some 200 to 250
militia from Livingston, Daviess, and Carroll counties, acting under
Colonel Thomas Jennings, (also sheriff of Livingston county) marched against the village. Assuming that an
earlier truce still held, the residents were surprised by the late
afternoon attack. Church leader David Evans' call for "quarter" was
ignored, and the villagers were forced to flee for safety. The Mormon
women and children fled south across a stream into the woods, while the
men gathered in the blackSmith shop, but found it a poor place for
defense because the Missourians were able to fire through the widely
spaced logs directly into the group huddled inside.
Seventeen Latter-day Saints and one friendly non-Mormon were killed.
Another thirteen were wounded, including one woman and a seven-year-old
boy. (ibid.)
This is event was even more heinous because of some of the murders, including that of children: "After the initial attack, several of those who had been wounded or had
surrendered were shot dead. Members of the militia entered the shop and
found ten-year-old Sardius Smith, eight-year old Alma Smith, and
nine-year-old Charles Merrick hiding under the blacksmith's bellows.
Alma and Charles were shot (Charles later died), and William Reynolds
put his musket against Sardius's skull and blew off the top of his head,
killing him.
Reynolds later explained, "Nits will make lice, and if he had lived he would have become a Mormon."
Seventy-eight year old Thomas McBride surrendered his musket to
militiaman Jacob Rogers, who then shot McBride and hacked his body apart
with a corn knife. Several other bodies were mutilated or clothing
stolen, while many women were assaulted. Houses were robbed, wagons,
tents and clothing were stolen, and horses and livestock were driven
off, leaving the surviving women and children destitute. (Wikipedia)