The morality is important, as we're talking about whether to prosecute and legal arguments can be made both ways. As I said, I do not agree that a person stealing a taser constitutes a deadly danger to the public. Even if the taser is considered a deadly weapon, there was no demonstrated intent to attack random people.
As this is not a clear cut case of justifiable homicide, the right thing to do is let a court sort it out. If the courts are too slow and expensive, welcome to another broken aspect of the criminal justice system that is in dire need of reform!