I have no experience about stun batons, but I can speak about the use of force
continuum. The requirements for potentially lethal self-defense response vary from state to state. In a few states, for instance, one can use lethal force to protect property. In some states, the only time one can legally use such force is when your "back is to the wall"- ie, facing a deadly threat with no ability to retreat. Typically, to warrant the use of deadly force, your assailant must have
the:
means
desire
ability
to inflict serious or deadly harm to you or your family. If you, for instance, are hunting in the woods, and you pass a fellow hunter, who gives you a friendly wave, that hunter has both the means (a firearm) and ability (within range) to hurt you. He does not, however, possess the desire. If you were recently in New Orleans, and a crazed man clenching a knife in his teeth was trying unsuccessfully to swim to your motorboat, that man would possess both the means and the desire to hurt you, but not the ability...etc.
Now, if you are facing someone twice your size, who appears to have a serious grudge against you, you have a "disparity of force". If you cannot retreat, this large individual would indeed seem to have the means (physical force), desire, and ability (he can apply the force he has to your body) to cause grievous bodily harm to you, and as such, lethal force could legally be employed.
Hope this helps,
John