It's truly a tragic story. I read other information regarding Mr. Johnson some years ago - IIRC including from one of his crew members.
The Wikipedia article makes it sound like he walked in on a robbery in progress, but the testimony of the store owner and the people waiting in the car for him, both indicate that he was the robber and that he was alone.
Robbery Try By Viet Hero Laid to Illness - The Washington Post
From Dakto to Detroit: Death of a Troubled Hero - The New York Times