Vampires don't die from old age; They aren't born, and all vampires were humans at some point in their life.
After being bitten a human can turn into a vampire (although the chance is very low, otherwise vampires would run out of food). They're NOT mindless monsters having their own personality, memory, and feelings too. It's clear that minds of new vampires aren't "tabula rasa", and they seem to have at the very least some memory and personality of their old selves. This makes it clear that something probably changed in their personality because vampires in general display attitudes that look down on humans (depends on individual vampire, so can vary) such as:
"Humans are cattle, vampires are the master race."
"It's natural for vampires to attack humans, so there is nothing bad about it."
"Humans will die someday anyway, so why bother about their lives?"
This is despite them being humans once and still having memory of human life.
How can I explain the radical change in personality when a person has been transformed into a vampire?