An ugly/disturbing aesthetic in a work of art might simply be achieved through the depiction of characters or scenes that are hideous and disgusting. The artist might incorporate art elements such as haphazard/weird distortions of form, splotchy texture, uncontrolled scumbling, sickly and muddy colors, erratic edges, and an off-balance composition.
Artists might manifest an ugly/disturbing aesthetic in a piece to express their reaction to horrific events or a traumatic experience. Artists might choose this aesthetic as a reflection of the character(s) or setting being portrayed. An ugly/disturbing aesthetic might also be adopted to make a piece suitable for a setting that demands something horrible.
Featured: Saturn Devouring His Son by Francisco Goya.