If a cop walks into a situation where a man has a gun to someone's head threatening to kill that person and then turns the gun on the cop, the cop has to shoot. However, if a cop knows that there is a man with a gun to someone's head inside a dwelling, which this cop did because the girl who escaped informed 911 of that fact, then the cop shouldn't enter the dwelling and provoke a confrontation. They aren't trained for that- that's what swat teams are for.
The question as to the guilt of the cop is whether he entered the dwelling to apprehend the suspect or whether the suspect inadvertently approached the cop.