I was reading about the origins of the word decimate and noticed a cool little nugget of historical cruelty.

A decimation was used to punish Roman soldiers for acts of cowardice or mutiny. When a cohort (a military unit consisting of 800-450 soldiers, depending on the time period) was to be punished, the men were divided into ten groups and lots were chosen. This was usually done by choosing colored pebbles from a sack. The man who chose the wrong colored pebble would then be executed by his fellow group members, usually by stoning or clubbing.

The reasoning behind this punishment was that since every member of a cohort, from the top to the bottom, was susceptible to being executed for the actions of other members, such punishment would hammer in fear, correct behavior and group resolve. Since the execution of one-tenth of a cohort is a fairly sizable amount of men, this punishment was rarely used except instances where extreme behavioral modification was warranted.