Readers are expected to make nnumerous inferences throughout Shirley Jackson's "The Lottery." Readers can ask is Tessie being late is ironic. Readers can question if the setting is meant to contrast the darkness of death. Readers may also question if Mrs. Delacroix chooses a rock so large that she can barely carry it in order to end Tessie's suffering sooner.
Essentially, the answer to the last question (and the one posted) is subjective. Some readers may feel as if Mrs. Delacroix felt sorry for Tessie, and she knew that a large rock would take Tessie's life quicker than a smaller one. Other readers may feel as if Mrs. Delacroix did not really like Tessie. If she did not like Tessie, a large rock would cause Tessie more pain.
Either way, a reader could justify either answer. Mrs. Delacroix could feel pity for Tessie, yet she (Delacroix) may not like her either. Readers simply are not told directly.