“From the first appearance of man upon the earth, down to very recent times, the words "stranger" and "enemy" were quite or almost, synonymous. Long after civilized nations had defined robbery and murder as high crimes, and had affixed severe punishments to them, when practiced among and upon their own people respectively, it was deemed no offence, but even meritorious, to rob, and murder, and enslave strangers, whether as nations or as individuals. Even yet, this has not totally disappeared. The man of the highest moral cultivation, in spite of all which abstract principle can do, likes him whom he does know, much better than him whom he does not know. To correct the evils, great and small, which spring from want of sympathy, and from positive enmity, among strangers, as nations, or as individuals, is one of the highest functions of civilization.”

Concerning the Jews (Harper's Magazine, Sept. 1899)

Imperial Adam (l. 42-44).

Source: The Broken God (1992), p. 278

To Leon Goldensohn, April 6, 1946, from "The Nuremberg Interviews" by Leon Goldensohn, Robert Gellately - History - 2004
Context: What I would like to emanate from the darkness of this tragedy is one spark of life. I mean, the realization that crime does not begin when you murder people. Crime begins with propaganda, even if such propaganda is for a good cause. The moment propaganda turns against another nation or against any human being, evil starts. Whereas the Germans started propaganda toward the end of this tragedy, you Allies stand at the beginning of the tragedy.

Letter XCV: On the usefulness of basic principles, lines 30-32.
Epistulae Morales ad Lucilium (Moral Letters to Lucilius)
Context: We are mad, not only individually, but nationally. We check manslaughter and isolated murders; but what of war and the much-vaunted crime of slaughtering whole peoples? There are no limits to our greed, none to our cruelty. And as long as such crimes are committed by stealth and by individuals, they are less harmful and less portentous; but cruelties are practised in accordance with acts of senate and popular assembly, and the public is bidden to do that which is forbidden to the individual. Deeds that would be punished by loss of life when committed in secret, are praised by us because uniformed generals have carried them out. Man, naturally the gentlest class of being, is not ashamed to revel in the blood of others, to wage war, and to entrust the waging of war to his sons, when even dumb beasts and wild beasts keep the peace with one another. Against this overmastering and widespread madness philosophy has become a matter of greater effort, and has taken on strength in proportion to the strength which is gained by the opposition forces.

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

1900s, Letter to Winfield T. Durbin (1903)

Source: Dragonfly in Amber