During its long history, the BIONICLE universe has been plagued by those who would sacrifice others for their own purposes. Thuggish Piraka, former warlord Barraki, Dark Hunters, Rahkshi, flying Makuta, spidery Visorak... these and more have made life difficult for Matoran and Toa alike.
Not all are actively evil - the Bohrok have a mission to fulfill, for example, as do the Vahki. That they were misused was not their fault, but they proved to be formidable nonetheless.