The ethics of using a taser on someone depends on circumstance. For law enforcement agencies, the taser is seen as another tool for effective subject control while maintaining safety for the subject, other people and the law enforcement officer.
With appropriate training, the law enforcement officer should have a clear idea of when it is appropriate to use a taser on someone. Therefore, use of a taser in an appropriate, justified manner would be ethical, eg, using a taser to subdue a person armed with a knife acting in a threatening manner. Any use of the taser that cannot be justified would therefore be unethical, eg, after subduing the person armed with the knife, continuing to electrocute the person just because you can. Once effective control has been gained over the person tasered, use of the taser should stop.
The taser is a better option for law enforcement than using lethal force (gun), and offers more safety all round than other options such as a baton or using physical subject control techniques. The use of the taser in an appropriate and justified manner will ensure that the taser is used in an ethical manner.