Civies aren't allowed to carry tazers or stun guns in my state. This is a point of controversy among my LE friends and trainers. Opinions vary on this.
I think that reasonableness due to circumstance is what should vary between civie and LE, not tools of force.
I am of the opinion that for what is true for an officer regarding carry and use of force should be true for the citizen. I ascribe to the "old west" philosophy that the police are citizens with trusted responsibilities that go with the badge, but are citizens just the same. And I believe that it is every citizens responsibility to do what it takes to protect themselves and their own communities, not just the police.
Considering this, I don't think that the police should be allowed to carry a tool of force that the public can't, and vice versa. If the police can carry tazers, then so should private citizens. Reasonableness isn't bound only to those with a badge or to those without. A civie would still have to use reasonableness if he/she used a tazer, or be prosecuted to the full extent of the law.
The reasonableness due to circumstance regarding use and carry of a tool is what should vary between officer and civie; but not ownership or carry of the tool. In other words, it would be reasonable for an officer to taze someone, if given fair warning, for not complying to reasonable verbal directives, as this avoids a physical altercation that could be injurious to both parties while making an arrest. In most circumstances, it would not be reasonable for a civie who's job IS NOT to enforce the law and who IS NOT making an arrest to taze someone for not complying with a verbal directive.
So, what varies is what is reasonable for an officer vs. a civie. What should not vary is the tool of force.
That said, using lethal force if under threat of a tazer should be considered reasonable under the right circumstance, for a civie or LE, as incapacitation could lead to severe injury or death. This is pending the totality of the circumstance, however. Obviously, I can't legally shoot an officer who gives fair warning of a pending taze because I am under arrest and I am not complying, where as a cop could shoot a criminal threatening him/her with a tazer, provided that it could be assumed that the intentions were to inflict grave bodily damage or death (by disarming the incapacitated officer and inflicting more damage, for example).
But, that is just what I think. Too bad that I think a lot of people will probably disagree with me on this.