Seeing 'mercy' clued in a puzzle and the grid filled with 'pity' made me grin right away. At its simplest, crosswords love short, common synonyms: 'mercy' and 'pity' overlap in meaning (compassion, sorrow for someone’s suffering), and 'pity' is a compact, everyday word that fits many themed grids or tight crossings. Puzzle constructors often pick the shortest, most familiar option to avoid obscure vocabulary or awkward inflections.
Beyond pure convenience, there’s nuance: 'mercy' can mean leniency or clemency when someone is spared punishment, while 'pity' leans more toward sympathetic sorrow. But when a clue is just a one-word, straight definition, the setter is allowed to choose any valid near-synonym that fits the pattern and intersecting entries. So if the down words force a four-letter answer like PIY, 'pity' becomes the natural entry. I like that little practicalism in puzzles — it keeps the solving flow smooth and the language relatable.