Harry Potter and the Deathly Hallows
Affiliation: Death Eater
OK, so if you haven't figured it out already, Bellatrix Lestrange is one crazy witch. Her years in Azkaban Prison seem to have only strengthened her love for Voldemort and his mission, and she's his most fervent, loyal follower. In this book, we see her in charge of her disgraced family (the Malfoys), and all she ever thinks about is Voldemort's success. She's unafraid to be cruel – in fact, she loves it – and we see her wield her famous cruelty against Hermione when our favorite trio is imprisoned in Malfoy Manor. To make things worse, not only is Bellatrix criminally insane, she's also arrogant as all get-out; her mockery is almost as bad as her malice.
However, it seems that this arrogance is what gets her in the end – she dies at Molly Weasley's hand during a furious final battle, at the moment that she's busy laughing at Molly mockingly. This reminds us briefly of her cousin, Sirius (who she actually killed herself), whose own downfall was his arrogance… perhaps a Black family trait?