Scratch a civilized person and you find a savage.

Literal

Civilized-person [even-も] one-skin peel-[if-ば] savage.

一皮むけば is a vivid idiom meaning 'if you peel off one layer of skin' — suggesting that the veneer is thin. The も after 文明人 means 'even a civilized person.' The sentence has a proverbial, cynical tone — beneath the surface, humans are still wild.