According to the description of the same vampires from the sixth installment, becoming a vampire is quite an honor for a Necromancer, but the elements of the ritual described differ from what Markel performed (since this in no way resembles a gift from Ashi). And yes, Giovanni was a human who was aware of his actions and became a vampire of his own free will, while he was still alive. As for Nicholas...
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plus, the plot of the sixth installment takes place before the plot of the fifth, so maybe at that time the Necromancers had different priorities