At the current Rubens exhibition at the National Gallery there is a painting called "Murder of the Innocents" which includes a figure of a man holding up a baby. He is about to smash its head on a stone that is already covered in blood. How can that painting be beautiful?
Perhaps it is terrible, awe-inspiring, terrifying, moving and so on, all perfectly appropriate aesthetic qualities. Or perhaps despite the subject matter it really is beautiful, and the beauty of the composition is magnified by its contrast with the subject matter.