There really is something particularly heartbreaking about watching Korra get beaten so easily. In previous seasons, she could have taken them out in seconds (probably with one hand!) but right now even that overwhelms her. The physical side of being the Avatar always came easy for her: that's something outright stated in the first season. And all these years later, something that was once so natural to her is a constant struggle. It's *really* sad, but a very realistic and moving depiction of what being traumatized/physically hurt that badly is like.