Kill Joint is a form of controlling the creature. It collides with the idea of the game pretty hard.
I don't think it's any different from placing a muscle somewhere; It's simply another mechanic added to the creature. I'm not talking about a joint that does anything on command to the user, but rather one that does something when it touches the ground. Isn't it the same kind of "controlling the creature" you get when you decide to add a bone to stop two joints from rotating?