• Aesthetic choices are made that impart an unattractive impression
An ugly/disturbing aesthetic in a work of art might simply be achieved through the depiction of characters or scenes that are hideous, and/or an artist might incorporate stylistic elements such as haphazard/weird distortions of form, splotches, uncontrolled scumbling, sickly and muddy colors, erratic edges, and a disorderly composition.
An artist might manifest an ugly/disturbing aesthetic in a piece due to his or her own personality, or the artist might choose this aesthetic as a reflection of the characters/objects/moment in history being portrayed. An ugly/disturbing aesthetic might also be adopted to make a piece suitable for a setting or purpose that demands something horrible.
Featured above: Saturn Devouring His Son
by Francisco Goya.