Mrs. Abbott was the mother of Hannah Abbott, a Hufflepuff in Harry’s year. Mrs. Abbott was found dead in the fall of 1996, having been murdered by Death Eaters (HBP11).
Although never stated, we can deduce that Hannah's mother is Muggle-born because Rowling has stated that Hannah is Half-blood (PC/JKR1). Hannah's father, Mr. Abbot, is from the Pureblood Abbott family, so it is likely that he was Pureblood himself. If so, Hannah's mother would have been Muggle-born for Hannah to be a Half-blood.
Perhaps it was her Muggle-born status which singled her out to be killed by the Death Eaters in 1996.