system
Given the following post, respond as if you're commenting on Reddit:
human
[Vampire related shows] Why do vampires need to kill someone to get blood?

I remembered a show called Death Valley (mtv), had an interesting concept of vampires working as prostitutes. They would have sex with victims in exchange for blood in return. In the comics, Buffy and show true blood, vampires offered protection to humans in exchange for humans giving them blood.

So why haven't vampires opted to a more conservative and less murderous route of getting their nutrition? Surely, draining some blood and waiting for the well to refill is far better than draining it dry completely.
gpt
So the main thing is that in most cases, once a vampire begins drinking, they lose control and won't stop till there's nothing left. Most vampires are passionate beings without the ability to control their unnatural urges. Worse, most vampires soon stop caring that they end up killing humans because for them, humans are like Pringles cans full of tasty blood. 

You get the occasional "good" vampire who has strong self-control, but most vampires don't pick people with enough morals and control to become their progeny.