here is a philosophical approach (=intuitive), i did not invent it, but this is how i can understand the world, since i cant rely on facts, since i have no memory for facts.
animal nature is a higher structure that plant nature. the higher the structure of a species, the higher the agency, that protects the uniqueness of the evolutionary branch. (kind of unfinished thought ...)
other idea:
protein should probably be seen as a product, not as source, especially not as a source of "live". eating animal protein is like eating the recycled source. the whole process of assimilating this recycled material is a low energy process, compared to the more difficult process of producing human protein from scratch (=plant).
the idea is, that live is a process, and living in a low energy process is like sitting on a chair all day long. producing human protein from scratch is like gymnastics. live is not taking things in. live is throughput. live is not the source or the product (human protein), its the process in between.
the higher (deeper) the process, the more unique it is, the more it has potential to make us human (=specific, =agent, =individuated).
if we use only a part of our potential, by chosing the easy process if given the choice, we may shrink in our potential, become vampires.