Angelus

Angelus is the evil half of Angel and a major antagonist in Buffy the Vampire Slayer - whenever Angel becomes truly happy he is transformed into Angelus, this is due to a curse put upon him by gypsies in the past in vengeance for his misdeeds.

As Angelus Angel is devoid of all human emotion, save perhaps cruelty, he is also a ruthless killing machine - in other words Angelus is as bad as any of Buffy's "Big Bads" and indeed has been described as one himself.

Angelus is powerful for a vampire and alongside Spike and Drusilla became one of the few vampires that assaulted Buffy and survived for more than one season, the conflict Buffy had with Angelus was much more personal than any of the other "Big Bads" due to the fact she was deeply in love with Angel - arguably Angelus has caused the most emotional pain to Buffy during the series and as such could be considered one of the most evil of Buffy's villains.

Of course due to the fact Angel is a hero within the Buffy universe the transformations into Angelus rarely lasted long but when they did occur it was always memorable and caused great tension within the group due to Angelus' liking of attacking his opponents on a very personal level.