Vulcans' natural and uncontrolled emotions are erratic and volatile; paranoia and homicidal rage were common on Vulcan prior to the adoption of Surak's code of emotional control.
Millennia before Earth's first contact, Vulcans were once an extremely violent and emotional people, even by Earth standards. Vulcans of that time waged almost constant warfare on differing factions of each other. As the level of technology improved, it was clear that extinction was a distinct, if not inevitable, possibility. It was because of this realization that Surak began teaching his philosophy of logic.