Have a look at my Vampires. Why did a guardian angel become a vampire? Because they want to be. They've become "Vampire Guardian Angels". They want power and immortality. They no longer want to serve man---they would rather eat man. For them, it's a lot better to become an evil monstrosity rather than a good servant to humans. Becoming a vampire makes it easier for them to get rid of humans, so they won't have to answer prayers anymore. As guardian angels, they could not kill human. But now as a vampire-angel hybrid, they can slice and dice and eat humans all they want.
Gloriously gruesome, aren't they? :)