If you mean the worst character in the series, not just the real Big Bad from each individual season, then my vote would have to go to the werewolf hunter from (I think) the second season. He knew that the creatures who he hunted and killed were human except for the three nights of the full moon, knew that he was, essentially, actually killing people, and he didn't care. He didn't do it for sport, for self defense, to protect the community, or even to collect any kind of personal trophy. He did it purely for profit, the single most indefensible motive for taking a life. Even the "gourmets" from Angel's Season 5, who kidnapped Nina, were at least going to eat what they killed.