Question though...did Carol stab the woman who was screaming after having been attacked? Or did Carol smother her? I think that woman had been cut with a knife...wasn't sure if she was going to die, so Carol did a mercy-kill...or if that woman's screaming was putting them both in danger, so Carol had to take her out. Either way, Carol did what she had to do.
I believe Carol gave the lady mercy because she was yelling in pain and was going to die anyway. Much like Rick gave the nosy guy (Carter?) mercy when a zombie bit his face off in the last episode.