Yeah.. their ability become a disaster after realize that they were hurt their friend or their family unconciously.. the legend said people with this ability must learn to control their temper. Coz one they say something, they cant take it back..
And i remember the scene in Ze, when Ryuusei (has kotodama ability) got angry and he said die to his mother, his mother died and he regret it to the point he dont want to use Kotodama anymore.
Have anyone ever read a yaoi manga "ze" by shimizu yuki..
The story about kotodama master that can curse people. Is it possible that the story was inspired by some urband legend.??. coz i heard about this legend some time ago and here It called "bitter tongue" coz when they got angry all they said will come true