Can anything in Buddhist theory explain phenomena like Buck Fever?
Angulimala (finger garland) Bandit was probably the most ruthless serial killer during Buddha's time.  I read somewhere but didnt do much research on it that in his previous life he offered food to previous buddha or arahat who was tired and weak from hunger, after that in every life, Angulimala had extraordinary strength than everyone around him.  in one of his lives he was a wild buffalo attacking the domesticated buffaloes.  The villagers got upset and set up a lynching mob to hunt him down.  He was eventually captured, tortured, and killed.  He wanted to take revenge on those who killed him.  Among the villagers, there was a girl who watched the lynching with teary eyes.   She became his mother in his last life (which woulda been his last victim if Buddha didnt intervene) and the villagers became his victims.  However, i dont think he took pleasure in killing but rather was misguided.     Kalayanamitra is certainly a blessing.