Doctor Weirick

Giles: "..how terribly frustrating for you, that a bunch of school children could accomplish what you could not"

Doctor Weirick: "it bothered me, but the power will be mine"

Doctor Weirick was the true antagonist of the Buffy episode "The Pack" - although he didn't reveal himself as such until near the end of the episode: it was due to his actions that Xander Harris and a gang of local thugs became possessed by the spirits of hyenas, turning them predatory in nature and highly dangerous.

History
Doctor Weirick worked as the zookeeper at Sunnydale Zoo but was also secretly a Primal, a cultist who believes that humanity is a perversion to nature and that true perfection exists via shedding humanity in favor of pure animal instinct: his ultimate goal was to become possessed by hyena spirits so as to ascend into what he believed was a higher-state of existence as a true predator. However he continually found himself unable to commit the necessary predatory act to complete the ritual - much to his annoyance four Sunnydale bullies and a hapless Xander managed to commit this act via fighting near the enclosure and became possessed by the hyena spirits.

Doctor Weirick was enraged and kidnapped Willow, determined to retrieve the power he had been obsessed with for years - when Buffy and her friends arrived Doctor Weirick, now dressed in full ceremonial gear, held a knife to Willow's throat and threatened to kill her on the spot, this savage act earned him the favor of the hyena spirits and he became possessed: however he was no match for Buffy who overpowered him and accidentally pushed him into the hyena pen where he was devoured by the very animals he had once worshipped.