Leonardo da Vinci's Ethical Vegetarianism a detailed study by David Hurwitz

. . . To this difficulty we have to add another one: the scarce or no importance which biographers pay to the eating habits of the people they write about. So for example Colin Spencer complains in his book The Heretics Feast that among 60 biographies about Leonardo Da Vinci only two of his biographers mention that he was a vegetarian. - Professor Luis Vallejo Rodríguez, Spain

• Leonardo da Vinci, artist, thinker and man of science Vol.1 (link to archive.org) by Eugène Müntz, 1898. p.17 footnote: "It appears from Corsali's letter that Leonardo ate no meat, but lived entirely on vegetables, thus forestalling our modern vegetarians by several centuries."

Truly man is the king of beasts, for his brutality exceeds theirs. We live by the death of others: We are burial places! - from da Vinci's 'Notes'

[note: the following quote is from a work of fiction written by Merijkowsky in the 1920s - it has been established that this is not an authentic quote. The attributions for the quote above and the one below were interposed in Jon Wynne-Tyson's book - and have been misquoted ever since.]
I have from an early age abjured the use of meat, and the time will come when men such as I will look upon the murder of animals as they now look upon the murder of men. - Merijkowsky, Romance of Leonardo da Vinci

Nothing will be left,
Nothing in the air, nothing under the earth, nothing in the waters.
All will be exterminated.
- from da Vinci's 'Notes'