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.