McCoy said that the Defiant crew killed each other. From http://www.chakoteya.net/StarTrek/64.htm
KIRK: Get back up here.
(Chekov has a dizzy spell, and totters out of Engineering.)
KIRK: Kirk to McCoy.
MCCOY: McCoy here.
KIRK: Bones, can you tell me what they all died of?
MCCOY [OC]: I'd say these people killed each other.
KIRK: They what?
MCCOY: You heard correctly, Jim. These people killed each other.
KIRK: Could mental disease have infected all of the crew?
MCCOY [OC]: According to the ship's log, the medical surgeon down here didn't even know what was going on.
MCCOY: The best I can do is get all the readings I can get and analyse them later.