She did the thing she was trianed to do. This really is an easy mistake to make. She was too focused on the situation to notice what her hands were doing. This is just like when you try to push your glasses up you aren't wearing them.

Reflex actions really are triggered by the lower motor neurons and can happen without conscious intention. And sufficiently trained skill can get internalized as a reflex like this. Having the stun weapon so similar to the kill weapon would easily allow for a person to get confused and do this below the level of human thought. Which is why cops both shouldn't have guns and shouldn't be so quick to do violence to organize the situation.