In the Hebrew Bible, God curses Cain after he kills Abel, and tells the Israelites that terrible curses will befall them if they do not follow His laws. Humans also place curses on themselves or others. Scholars of the ancient Middle East have come to understand such curses, which were no less common among the Israelites’ neighbors, as an important part of ancient legal systems, and also as a form of prayer, as Anne Marie Kitz writes (free registration required):
Oaths required petitioners to call upon the deities to punish them should they lie or be unfaithful to the terms of a contract. Such conditional self-curses were not taken lightly. This was especially the case because people were typically made to swear on a weapon that purportedly belonged to the deity. Should an individual violate the oath, the weapon would be used to execute the penalty, usually death, as an expression of divine judgment. . . .
Curses and blessings [are] nothing other than prayers uttered by mortals to the divinities. They are neither commands nor demands, and there is certainly no assumption on the part of the speaker that either will have instantaneous effect. In the end they are little more than strongly articulated wishes. Deities, on the other hand, articulated curses differently. As supreme beings they did not need to invoke a higher power to enact a malediction. Their curses were commands that mortals believed had immediate consequences.
Read more on ASOR: http://asorblog.org/cursing-in-the-ancient-near-east/