Psychopaths

Generally, when a zombie apocalypse happens some survivors become insane do to the unexpected nature of outbreak. When that happens, they loose all sense of understanding and will go as far as attack the living. Highschool of the dead has a small amount of Psychopaths (4 total), although they are not officaly named so, and about half have proceeded to sexually assault Rei Miyamoto and Shizuka Marikawa.

Threat
When its comes down to Psychos, they can be just as dangerous if not worse as Them. A majority are seen brandishing a weapon (mostly knives).

Insanity
Despite the small number of psychos, some characters show signs of insanity. This usually happens when one experiences too much emotional pain and can't pull him/herself together or feel as if they are "king of the world" (the later is due to societal collapse and lack of moral code). Those who go insane can become a psycho, but some, and most often, can regain their sanity.

Psycho survivors

 * Thug (died and became one of Them).
 * Shimada (redeemed and sacrificed himself to save a survivor).
 * A tattooed thug who was firing his shotgun at both zombies and other survivors.
 * A butcher who was swinging his butcher knife at both zombies and survivors.
 * A survivor wearing businessman-like short-sleeve dress shirt and dress pants who also was shooting his own shotgun at both survivors and zombies.
 * The two teens from the Taiei shopping mall (one died, the other regained his sanity and escaped once Shimada sacrificed himself).

Insane survivors

 * Takashi Komuro (experienced its when raiding the gas station and sometimes when fighting Them)
 * Kohta Hirano (gets evil look on face when looking at guns, fighting Them, and brokedown after killing Asami Nakaoka)
 * Saeko Busujima (nearly killed a man 4 year prior to events of the series and enjoys killing Them. Note: In the English dub, her name sounds and pronounces like the word "psycho")
 * Saya Takagi (Broke down after she killed one of Them with the drill)