Quoted allegedly "From da Vinci`s Notes" in Jon Wynne-Tyson: The Extended Circle. A Dictionary of Humane Thought. Centaur Press 1985, p. 65 books.google http://books.google.de/books?id=1mMbAQAAIAAJ&q=murder.
Actually the quote is not authentic but made up from a novel by Dmitri Merejkowski (w:Dmitry Merezhkovsky) entitled "The Romance of Leonardo da Vinci" (La Résurrecton de Dieux 1901), translated from Russian into English by Herbert Trench. G.P. Putnam's Sons New York and London, The Knickerbocker Press. There, in Book (i.e. chapter) VI, entitled The Diary of Giovanni Boltraffio, one finds the following:
The master [Leonardo da Vinci] permits harm to no living creatures, not even to plants. Zoroastro http://it.wikipedia.org/wiki/Tommaso_Masini tells me that from an early age he has abjured meat, and says that the time shall come when all men such as he will be content with a vegetable diet, and will think on the murder of animals as now they think on the murder of men ( p. 226 books.google http://books.google.de/books?id=g_pa0OaYX64C&pg=PA226).
However, despite the quote's false attribution, da Vinci was in fact a vegetarian.
Misattributed
