I appreciate the length of this post will put some people off of reading it but if others wish to dissect the material into individual points for scrutiny feel free.
As animals ourselves we are designed to be able to survive on many kinds of food (it was thought that this was because we were largely a scavenging creature until developing our hunting skills and eventually agriculture).
Pigs and chimps have a similar body structure so much so that's it's possible to transplant bits of pig into humans and vice versa and have those bits continue to function. My uncle had a pig valve in his heart for years.
As such the Western cultural obsession with eating mostly animals is certainly not healthy. And more akin to the diet of 17th Century nobility (many of which had the same health problems as modern Westerners).
Thanks to globalism this is spreading to other places of the world and changing the bodies of peoples not used to eating so much animal protein. The stereotype of people from the orient being short was largely accurate until very recently where the change of diet caused by adopting a more Western life style has lead to taller, fatter people who are more prone to heart disease.
If you investigate the physique of feral or hunter gather people alive today or people generally in the past you see most of the health risks we face barely existed. People tended to live shorter lives in the past but that was down to perils we have so far successfully controlled.
I mention all this because it goes to show one thing. That we are animals like any other. The only differences are cultural and that is largely down to becoming slightly more sophisticated at tool making and use than a crow, around the time of the beginning of the current ice-age.
By developing drawing which later turned into spoken and written language we became able to order thoughts and record them for generations.
Rather than acting on instinct we had another layer of complication over our lives, tradition. Traditionally the successful ate meat regularly because they had land and servants to produce a constant supply. The poor would use a cow to pull a plow, poo everywhere to fertilise the soil, to give milk and calves and would only eat it when it was close to death and had birthed a successor. They would sell the excess animals where they would either be eaten by the rich or enter into a similar arrangement with another poor family.
New World farming methods threw that order aside so now anyone could get access to cheap meat because land was plentiful and eventually technology would replace some of the other uses for a cow. But because the human animal has the cultural aspect it still equated meat with success and strength despite our ancient and stronger ancestors living on a largely cereal based diet.
The animals you see on a modern farm are created by human civilisation too and they bare little resemblance in physique and action to their 'natural' cousins.
The landscape of Europe is created by the human civilisation with miles and miles of natural forest felled to create pasture for edible sheep. Modern methods have removed the natural flowers that wild bees fed on and the trees they grew their colonies in. Most bees you see now are kept bees or solitary bees.
One can make a very good case for human civilisation being an evolving life form of it's own, adapting, evolving placing some 'nodes' over others in value and possibly eventually removing the need for us and escaping to the stars.
For me the a speciesist approach to attributing value to life should be as out-moded as a racist approach.
In the past white Europeans sent hunters to find and decapitate non-white 'exotic' people for their comparative collections (like their collections of insects and birds eggs the hunting for which brought many species to extinction). The use of black Africans as slaves of course is written into the DNA of American culture as is the genocide and displacement of Native American peoples.
The excuse used for this was that they were not entirely people, they lacked the right customs and religion and they were superficially different. The Nazi's described Jews as 'vermin' and disposed of them using the techniques usually used in taking cattle to slaughter. They were branded, crammed into trucks, shaved, worked to death and processed into soap and lampshades etc.
We are rightfully horrified by this because these were people like us who's only crime was being seen as different enough to treat as objects to use or remove.
A dog can be seen as a beloved friend and yet it's near cousins foxes, wolves, dingos etc are hunted because they are an inconvenience. Some cultures turn their noses up in disgust at eating cats or horses while others see it as perfectly natural.
To me it makes more sense to not look at the outside of something but at the actions of something to build empathy.
I grow vegetables and were possible give them the space to self replenish so I harvest to make room for the plant to grow again the next year. I eat fungus and cereals with the expectation that the farmers who grow these things also replenish what has been reaped from the previous generation. I don't eat animals because I find it difficult to determine the point where one animal becomes significantly enough unlike (without falling back on the superficial criteria used by slave traders and the like) the animal side of me to not feel empathy.
I don't see any difference between cruel to a cat and being cruel to a spider. If I can remove either from danger I will attempt to do so.
It may be difficult to prove distress in a prawn but no more difficult that proving distress to an early term human fetus.
I am aware that my actions cause the deaths of hundreds of creatures though but there are things I have to do like walk and breath and actions I am culturally and domestically expected to do, like mow the lawn which will kill beetles and spiders and ants etc despite my attempts to avoid doing so.
In that awareness I attempt to find ways to minimise that damage by not bleaching the paving slabs, using weed killer, rodent killer, slug pellets or insecticide sprays.
Sometimes however the action of a life form endangers my well being beyond my ability to move it aside or live around it and action has to be taken. I recently took a course of antibiotics to cure a bladder infection for example and once had to clear the air vent of my house of an infestation of millions of small spiders that were eating the ceiling.
I felt bad for the spiders particularly as they were bought in by starlings on nesting material and we wouldn't have disturbed the birds if we could (it's currently illegal to do so anyway though I beloved Government is trying to change that for the benefit of their sponsors in the building industry).
I can't make anyone else act like myself and clearly I'm having a problem getting some people to understand myself but empathy is the element of consciousness that prevents people from selfishly grabbing whatever they want from life and if you can extend that to creatures with different body shapes extending it to members of your own species with different cultures, skin tones and ideas is much easier.
It will also help when your alien overlords finally reveal themselves.