A 17-year-old high school student, who confessed to stabbing to death his 12-year-old brother and parents, had been suffering a general sense of “malaise,” Italian prosecutors said Monday, adding that he has been unable to provide a motive for a triple homicide that has shocked Italy. The minor, whose name was being withheld because of his age, was being held on suspicion of murder with aggravating circumstances including premeditation. The youth took a knife from the kitchen and first entered the room he shared with the younger brother and stabbed him, according to prosecutors.