Cultured Badass: Melmoth. He's conversant in politics and culture, and he doesn't hesitate to murder to get his way. On the other hand the reader may find his gloom and desperation pathetic.
Memetic Mutation: The novel copies a popular theme of selling your soul to the devil, found earlier in Goethe's Faust and Matthew Lewis' The Monk. The Wandering Jew element was also copied from folklore.
Once Acceptable Target: The novel is partly set during the Spanish Inquisition and the sadism of the priests is certainly very anti-Catholic by today's standards. But during Maturin's time there was anti-Catholic sentiment among the Anglicans (Maturin was an Anglican clergyman).
Vindicated by History: The novel was reviled in England because critics thought that it was anti-Christian due to the evil hero. This affected Maturin's chances of promotion. Years after his death, authors admired him and were inspired by him. Oscar Wilde even used the pseudonym of Sebastian Melmoth. The psychological complexity and emotional intensity, unusual for his era, was praised by later novelists. Today, critics consider it the greatest novel of the Gothic school.