A huge red flag Voldemort should’ve noticed about Snape: Dumbledore knew Snape was a Death Eater, yet still allowed him to teach at Hogwarts. Voldemort knew Dumbledore was smart. So why did he never stop to ask himself: why would Dumbledore trust Snape? Voldemort assumed Snape was simply a brilliant spy. What he could never understand was that Snape might have genuinely changed sides because of Lily. Voldemort understood power, fear and ambition. He never understood remorse, love or redemption. And that blind spot is exactly what Snape exploited.