The Mari homeworld was a peaceful Delta Quadrant planet whose telepathic inhabitants had made violent thoughts a crime, and where B'Elanna Torres was arrested in 2374 over a moment of anger.
The planet is the setting of the fourth-season episode Random Thoughts. The Mari have almost eliminated crime by policing the mind, and the crew find their markets safe and welcoming.
When a passer-by bumps into Torres, her flash of temper is picked up by a Mari man who then beats someone. Chief Examiner Nimira holds Torres responsible and sentences her to an engram purge, a procedure that risks brain damage. A murder follows that appears to stem from the same thought.
Tuvok investigates and finds a hidden trade in forbidden imagery. The merchant Guill deliberately provoked Torres to harvest her anger for sale. Tuvok overwhelms him with his own darker memories, and the purge is halted.