She never stepped into the wrong apartment. She was trying her key in what she though was her lock when a black man opened the door from inside.
So she shot him.
Wouldn’t you? (do I need a sarcasm tag?)
I think that’s what happened. But they startled one another. He may even have been trying to leave as she pushed in and the physical contact frightened her more so she reacted mistakenly and shot him. She was in uniform, so she probably had her service revolver handy.