I was born into a vegetarian family. I still am a vegetarian, but I accept that one species eats another in this world and that is how nature has evolved.
What has stopped me from ever eating meat was this: if we encountered an alien race which slaughtered and consumed humans the way humans consume other animals, how would I feel? The answer was always that I would feel disgusted/horrified/angry.
Shouldn't we all feel the same about another sentient species? We could argue forever about what 'sentience' means, but beyond a certain point it just feels callous to not stop.