The horror isn't in the slaughter (although it's not pretty, especially in some operations) — the horror is in the factory farming operations where the living, breathing, conscious animals are treated like machinery.
As to Einstein:
It is my view that a vegetarian manner of living by its purely physical effect on the human temperament would most beneficially influence the lot of mankind.
From a letter to Harmann Huth, 27 December 1930
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