All savageness is a sign of weakness.
Omnis enim ex infirmitate feritas est.
Original text (Latin)
Notes from Wikiquote
De Vita Beata (On the Happy Life): cap. 3, line 4 | As quoted in Caxtoniana: A Series of Essays on Life, Literature, and Manners (1864), Harper & brothers, Sir Edward Bulwer-Lytton, p. 174 (in the essay The Sympathetic Temperment).
Variants
All cruelty springs from weakness. (translator unknown)