Disclaimer! I have not seen the leaked episodes, so my discussion is relevant up until the end of LoK book 3, episode 3. The rest is merely my speculation. There are no spoilers present in this blog.
Since the initial airing of the 'Puppet Master' years ago, the ideals of bloodbending has been one which has intrigued Avatar fans for years. Initially hallowed as the bending art of healing and life, waterbending suddenly was rendered in a dark and malevolent light, with bloodbending being reminiscent of controlling a person like a 'dark puppet master', as put by the Gaang.
With this in mind, I draw you to the Legend of Korra, book 1 antagonist, Amon, who learnt the technique from his father Yakone after the technique had both been put into the public eye and overtly outlawed by the now aged Katara. This poses a question. Why make bloodbending illegal if Hama was the only known user during The Last Airbender, who was soon made redundant before the 100 year war came to a close? Katara, as we all know, is an intelligent woman, and would be no means unnecessarily draw attention to such a dark art without proper reason. One could argue that Katara outlawed the skill after the initial reports of Yakone so as to deter the latter, but I have another theory.
Ming-Hua, the now armless waterbending prodigy, tried to capture the Avatar alongside her allies 13 years prior. Using her powerful waterbending techniques, Ming-Hua employed the use of bloodbending in attempt to facilitate her kidnapping of korra so many years ago. After having failed, the White Lotus acknowledges her dangerous potential, arguably being a greater threat than Yakone, and they decided to remove her arms and keep her locked away in her personalised prison so as to prevent her from bending, since there was no other means of removing her abilities available to the Gaang (with Aang's passing). Stories of this nefarious waterbender and her abilities spread, alongside those of the infamous Yakone, forcing Katara to make bloodbending illegal AFTER it had inevitably become public.
What are you're thoughts?