There's GOTTA BE more to the story....how do you enter the wrong apartment, not realize you're in the wrong apartment, encounter a person who RIGHTFULLY lives there, draw your weapon, still NOT realize you're in the wrong apartment, decide to shoot someone....
Upon entering, if the door was unlocked, and you assume someone was in the apartment, and decide to draw your weapon, you should be conscious of your environment. At that point you should be able to recognize the familiarity (or not) of the residence....
Unless the victim was just standing in the doorway, and startled her, why would she immediately assume a threat?
So many questions that need answers...