Is the makiwara is used to train the karate-ka to have a one-punch killing capability?

Let me begin this answer by expressing this emphatically, "This is my viewpoint, my perspective and my belief so take it with a grain of salt."

The makiwara is not a one punch killer tool for karate. This particular tool has many facets and many purposes where I can say one singular function of this tool is to assist karate-ka in learning and applying the fundamental principles of martial systems. It is a fundamental basics tool for fledgling karate-ka as well as an ongoing training tool to remind those more experienced practitioners to remain focused on the fundamental principles and the systems basics. One transcends any one system while the second brings a system to the forefront of the person's mind - a basis for familiarity of all systems.

Second, although I believe attempting to develop the mind, body and spirit for the spirit of one stopping power of punches and kicks I have to say that the ability to actually apply and achieve a one-punch stopping applied in a fight or violence virtually impossible accept in a fluke of a situation - much like attaining a hole in one, cool and good if you are lucky enough to get one but rarely done.