Brian de Bois-Guilbert is actually kind of unusual as a villain in a popular novel. We said in "In a Nutshell" that Sir Walter Scott was the J.K. Rowling of his day, responsible for the bestselling books of the early 19th century. That would make Bois-Guilbert his Voldemort, which doesn't seem quite right.
Bois-Guilbert is the main villain of Ivanhoe, he has a complicated backstory with our hero, and he certainly thirsts after power. But where Scott could have gone big with his central villain, giving him essentially no redeeming characteristics or sympathetic qualities (and a snake face to prove his inhumanity), Scott chooses instead to make Bois-Guilbert a surprisingly complex character.
Don't get us wrong: Bois-Guilbert has some crazy, Voldemort-worthy plans. When he's trying to persuade Rebecca to run away with him, he tells her that they could take over the whole of Europe and the Middle East with the army of the Knights Templar at their backs. That's a power-hungry fantasy equal to those of any insane dark wizard.
The main difference between Bois-Guilbert and a lot of standard bad guys (and super different from Voldemort) is that he has the capacity to love. He is proud, arrogant, and violent, but he loves Rebecca enough to offer to give up his place with the Knights Templar and join up with their enemies, the forces of Saladin's armies. He's willing to resign everything he has fought for and to start again in a less prejudiced place if Rebecca will go with him. That's a sign that he has genuine and deep feelings for her.
On the other hand, Bois-Guilbert frequently insults Rebecca's people, the Jews. He also physically carries her off from Torquilstone, despite her strong resistance. He doesn't rape her, but he makes it very difficult for her to say no. Only Rebecca's remarkable stubbornness and religious faith keep her from getting worn down by Bois-Guilbert's demands. His refusal to listen to Rebecca is what gets her into the whole witchcraft trial in the first place: because Bois-Guilbert brings her to Templestowe, she falls under the power of Lucas Beaumanoir. So he may "love" Rebecca (according to his own definition), but he doesn't treat her as though he truly feels for her.
If you feel like you can't get a read on Bois-Guilbert's emotional state, don't worry: he himself can't seem to decide where his passions lie. At the start of the novel, we learn that Bois-Guilbert is fierce and proud. He hates the Saxons and seems to take outright pleasure in insulting them. When Ivanhoe defeats Bois-Guilbert at the tournament, he is so furious that he insists on a rematch, anytime, anywhere. Bois-Guilbert is definitely arrogant and self-involved – his first love appears to be himself.
Bois-Guilbert later confesses to Rebecca that he once fell in love, back in France, with a woman named Adelaide de Montemare. He went on the Crusades and worked hard to build up his reputation to make himself worthy of her. When he returned to England, though, he found that she had shacked up with some small-scale landowner from the neighborhood. All the glory he won in her name was for nothing. Heartbroken and furious, he took a vow never to love a woman again.
That might have been the end of it, but Bois-Guilbert catches a glimpse of Rebecca taking the wounded Ivanhoe from the tournament to her house for medical treatment. He falls for her at first sight (so much for the oath). He takes advantage of the battle at Torquilstone to kidnap her and bring her to Templar headquarters at Templestowe. Although Rebecca clearly believes that Bois-Guilbert will rape her, he doesn't. He wants Rebecca to want to be with him. He offers her what (to him) seems like enormous compromises: together, they can leave England and join the Muslim troops in the Crusades. Bois-Guilbert will give up his family name and start rebuilding his reputation from scratch. He'll do all this if Rebecca promises to join him. This is yet another side of Bois-Guilbert's personality: the devoted, self-sacrificing lover.
But the saga of Bois-Guilbert's feelings is still not over. When Lucas Beaumanoir names Bois-Guilbert as the Templar champion in Rebecca's trial by combat, Bois-Guilbert seems set to refuse. Then Albert Malvoisin appeals to Bois-Guilbert's pride – and it works. Bois-Guilbert thinks, oh yeah, I am awesome! How can that smug woman reject my honorable offers to run away with me? I'll show her! And he suits up and gets ready to fight against Rebecca's champion and in favor of Rebecca getting burned alive.
It's not all that surprising when Bois-Guilbert winds up dying during Rebecca's trial by combat as "a victim to the violence of his own contending passions." Bois-Guilbert has waaaay too many contradictory feelings: his own arrogance, his hatred of women, his love of Rebecca, his anti-Semitism, his resentment of Lucas Beaumanoir, his pride as a Knight Templar... We're actually surprised he made it through Rebecca's trial without exploding from the force of all of these conflicting emotions. The stroke or heart attack that hits him when he's fighting Ivanhoe is certainly sudden, but it makes sense when you start thinking about his character. Bois-Guilbert's real battle is against himself, against his instincts to be a better man than he is. Unfortunately for him, he seems to lose that fight.