There's something in the DE mentality that appeals to the human mind and always has.
One aspect of Voldemort's appeal that tends to get overlooked is that he is implicitly offering his followers the hope of some kind of eternal life. After all, if he can achieve it himself, the suggestion is there that they might be able to follow in his footsteps (or at least the 'faithful' who have stayed with him all along) The DEs are a religious cult, in a way, which makes them much more dangerous than just a political movement, and explains their hold over someone like Snape, who I can't really imagine taking the 'pureblood' propaganda too seriously (given that he isn't one himself). I think for Snape and Bellatrix at least, its all about gaining the sort of magical power that Voldemort has for themselves, at whatever cost.
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One aspect of Voldemort's appeal that tends to get overlooked is that he is implicitly offering his followers the hope of some kind of eternal life. After all, if he can achieve it himself, the suggestion is there that they might be able to follow in his footsteps (or at least the 'faithful' who have stayed with him all along) The DEs are a religious cult, in a way, which makes them much more dangerous than just a political movement, and explains their hold over someone like Snape, who I can't really imagine taking the 'pureblood' propaganda too seriously (given that he isn't one himself). I think for Snape and Bellatrix at least, its all about gaining the sort of magical power that Voldemort has for themselves, at whatever cost.