As with most, if not all, conflicts between American Indians and English colonists in the seventeenth century, the cause of the Battle of Mystic on May 26, 1637, was not a single event but a confluence of events and beliefs.
Perhaps the most powerful reason for the battle is that the colonists, who were part of the original Massachusetts Bay Colony, believed that they had a divine right to populate the New World and to use the land--even though that land was occupied by indigenous people--for that expansion. In simple terms, from 1620 onwards, English colonists, who needed to expand their territory in order to accommodate increasing numbers of arriving colonists from England, began to take and use Indian land, and this included moving into what is now southeastern Connecticut. The constant encroachment by English colonists literally squeezed the Pequots, the main tribe in the area, into smaller and smaller areas, disrupting the Pequots' hunting and agriculture.
The immediate cause of the Battle of Mystic was the Pequots' killing of a trader named John Oldham, whom they believed was trading dishonestly with them. Because the Pequots had no recourse to a legal system to bring Oldham to justice, they used violence to settle their dispute with him. The Massachusetts Bay Colony then sent a force under John Endicott to bring the Pequots to "justice," and this action led to the later Battle of Mystic in which a force led by John Mason and John Underhill destroyed the Pequots' main village and killed as many as 600 men, women, and children, many of whom burned to death within the walls of their village.
In sum, then, even though the Battle of Mystic can be said to have been caused by the Pequots' murder of an English colonist, the true cause was a much more complex blend of beliefs, on the part of the English, that the land was their's to use by divine right; that indigenous people, because they were not Christian, were savages with no rights; and that the English had an absolute duty to spread the influence of the English people in the New World.