The police officer says that the young man was holding a gun. So what? He was in his home, he's allowed to hold a gun. The officer does not say that he was pointing the gun at her, nor that he was showing an aggressive tone that might indicate he might actually shoot.
This is a case of an incompetent officer using poor judgment. Pity the most that will happen is an administrative punishment. This person should be tried for 2nd degree murder.