Homo homini lupus est is a latin phrase meaning "man is a wolf to man." First attested in Plautus' Asinaria ("lupus est homo homini"), the sentence was drawn on by Thomas Hobbes when he wrote "Man to Man is an arrant Wolfe" in the opening line of the De cive, Epistola dedicatoria section of Leviathan as a concise expression of his view of human nature. The phrase is sometimes translated as "man is man's wolf", which can be interpreted to mean that men prey upon other men.

It is widely referenced when discussing the horrors of which humans are capable.
http://en.wikipedia.org/wiki/Homo_homini_lupus

It is a dog-eat-dog world.