God had promised Cain that no man may kill him, for whosoever killed Cain would suffer sevenfold.
For God's punishment upon Cain was to have him wander the Earth forever, not to take his life.
And as descendants of Cain and Lilith, vampires, carrying the blood of Cain, also inherited this promise from God.
However, vampires are merely descendants flowing with the blood of Cain, not Cain himself.
If it were Cain himself, of course the angels would not dare to kill him, but if they were just vampires, the angels did not care.
In fact, killing a vampire indeed comes with a heavy price. Van Helsing nearly lost his own life in the act of killing Count Dracula and was tormented by pain for the rest of his life, but it was not as Gilles de Rais had said, that the children of Cain could not be killed.
Gilles de Rais's words were merely meant to provoke the arrogant and conceited angel before him.
