Which character who has no martial arts training is the most lethal?
Lets divide it into scheming abilities and direct abilities such as medical knowledge of poison. Scheming needs a bit of clarification, being able to tell someone to kill someone else doesn't count unless they would normally never do it, but convincing someone towards their own end does count.
Madam Ma is the first person that comes to mind in terms of scheming.
As for direct abilities, poison is the only one that comes to mind, and most of the physicians in wuxia know atleast rudimentary martial arts.