No different than humans, when you think about it.

Killing animals and plants to sustain their existence...

Fact of life is, you're either going to eat a meal or be a meal, no matter what you are.

And the longer you exist, providing you're not immortal, the more lives will have to end just to sustain your own.

Vampires just go about sustaining their existence differently is all...nothing truly selfish about it.