Negligence - failure to exercise the degree of care expected of a person of ordinary prudence in like circumstances in protecting others from a foreseeable and unreasonable risk of harm in a particular situation
Accident - an unexpected, usually sudden, event that occurs without intent or volition although sometimes through carelessness, unawareness, ignorance, or a combination of causes and that produces an unfortunate result (as an injury) for which the affected party may be entitled to relief under the law
Mistake - an unintentional error.
Call it a mistake, negligence or accident. Accidents are typically caused by negligence or mistakes. I cannot say whether she intentionally or mistakenly drew her firearm. Firing the firearm, under the circumstances, as she believed them to be (assuming she believed the taser was in her hand, was undoubtedly negligent and I would go as far to say grossly negligent. Her calling Taser taser taser would lead me to believe that she did believe she had her taser out but was mistaken.
Regardless, 2 lives have been ruined.