Untimately: The Varieties of Fatal Experience.
For the record, Heroes Against Darkness has these pretty straightforward death and dying rules:
• Unconscious at 0 HP
• Dying from -1 HP
• No single attack can take a character to less than -5 HP
• Dying character loses 1HP/round
• Death occurs immediately at -10 HP
The -5 HP clamp for a single attack is because character and enemy damage in Heroes Against Darkness scales at higher levels, so characters can end up taking enough damage to take them from quite alive to dead-dead without this rule.