There are things in the breast of mankind which are best
In darkness and secrecy hid;
For you never can tell, when you’ve opened a hell,
How soon you can put back the lid.

Rudyard Kipling (1865-1936) English writer

“The Sons of the Suburbs” (1916)
    (Source)

On bloodthirstiness in war by previously peaceful people.

Originally written for the Christmas 1916 issue of Blighty, a magazine for servicemen. It was rejected, eventually to be published in the Sunday Pictorial (19 Jan 1936). It was never included by Kipling in any of his collections.