But it was just an accident. People do get confused and walk into the wrong apartment. It happens. It's not her fault that the occupant of the apartment was such a dangerous looking individual that she felt like her only option was to shoot him without thinking. Natural reflex.
Which is why she is being charged with manslaughter and not murder. And charging her with manslaughter makes it much more likely that she will be convicted since the prosecution doesn't have to prove there was an intent to kill.
which could amend the charges against her, up to first-degree murder.
Which would be a mistake. A charge of manslaughter here is pretty much a guaranteed conviction, while charging her with murder will make it easier for her to beat the charges. That was the mistake the prosecution made with Zimmerman. If they had charged him with manslaughter instead of murder, that trial would have likely turned out much different.