The latest woeful lesson in gun mayhem is playing out in Michigan, where a 54-year-old man in Dearborn Heights was charged Friday with second-degree murder in the shotgun slaying of a 19-year-old woman presumed to have knocked on his door in the middle of the night, seeking help after a traffic accident.

The Wayne County prosecutor, Kym Worthy, discounted claims that the man, Theodore Wafer, was within the bounds of the self-defense law, which requires his sensing a grave and imminent threat. There was no sign of forced entry and the victim, Renisha McBride, stood on the porch when she was hit in the face by a shot fired through the locked screen door of Mr. Wafer’s house, the prosecutor said.

Although there is speculation that Ms. McBride, who was black, was a victim of racial profiling by Mr. Wafer, who is white, Ms. Worthy said, “race is not relevant.” Neither was Ms. McBride’s state of intoxication a factor in the charges, the prosecutor added. Police said Mr. Wafer claimed he thought someone was trying to break in and the shooting was accidental after he opened the main front door to investigate. That doesn’t explain why he did not call 911 or why he opened the door with a shotgun in hand.

Whatever the outcome of the case, the tragic death of Ms. McBride was another symptom of a gun culture where private citizens are too often heedless of gun safety.