dragon age 2 quest act of mercy

hello i dont know did i posted question or not hello i hawe question why do i hawe to save grace the blood mage from the cave why u didnt give the option to kill her there why did we had to leave her allive for kidnapp my sister and then try to kill me what is the point of this quest even why do i hawe to save her can anybody anwser it please is it possible not to do this quest i mean do all other quest but not this because evrytime i see her ugly face i want to kill her in cave but there is no option tell me bioware why did u do this quest explain it to me im stupid guy who dont understand why i hawe to save that grace who later is gonna try to kill me ty but in other way i like dragon age and i like dragon age 2 i give your game 10 of 10 points