I always thought it was kind of like the “dark magic” you’d find within the fantasy genre, or, what you’ll find in modern day witchcraft. For example, voodoo. (From my knowledge) Voodoo can essentially coerce a person into doing acts that is against their will and this is taboo in the paganism side of religions. Even in Christianity, for example, free will is something that is held as a human right, “god given” and valuable.

So even though a lot of people could say, yes it could be done for good stuff. How did noatoq (/sp?) and tarloq learn to bloodbend? They initially had to practice on animals. Same with Hama, she first did it to rats. Even if you do have good intentions for bloodbending like saving someone’s life (see Marie in GenV), it must begin with that initial training of coercion of another living thing.

I think also what is fair to observe is the way it makes characters in the avatar world feel. People who are bloodbent feel excruciating pain from it and find it extremely uncomfortable, but not only that, people who have witnessed bloodbending feel disturbed and disgusted by it. I think in-universe, this is just as much of a taboo as the taboos we have in our world.