functionally insane
A completely unreasonable person who constantly reacts with anger or any other extreme emotion when they do not get their way often over simple things "normal" individuals take in stride, but not insane enough to be placed in a home (due to frequency of occurences) but still causes enough misery and grief for themselves and others around them.
Melissa made us all uncomfortable when she kicked the walls and struck herself repeatedly when she learned a check she wrote bounced at the bank. A simple error drove her into a rage and we all believe she is functionally insane.