Throughout human history there are terrible deeds inflicted on one group of people by another. These terrible deeds are lessons which should never be forgotten as they mark the path which is the moral ascent of mankind.
As ever, Robert Burns had some eloquent words to express this:

'Many and sharp the num'rous ills
Inwoven with our frame!
More pointed still we make ourselves
Regret, remorse, and shame!
And Man, whose heav'n-erected face
The smiles of love adorn, -
Man's inhumanity to man
Makes countless thousands mourn!

(from the poem: Man was made to Mourn, 1785)

The bitter irony is that Burns when down on his fortunes almost emigrated to Jamaica to take up a position as manager of a Slave Plantation.