Shamar Williams is accused of opening fire on 47-year-old Marcin Iwanowicz, a Polish immigrant.
Authorities say officers discovered Iwanowicz with a gunshot wound to the chest in front of Powell Street and Belmont Avenue just before 10 a.m.
EMS took Iwanowicz to One Brooklyn Health-Brookdale University Hospital Medical Center, where he died.
The shooting happened outside of the St. Jude Clinic, where Iwanowicz had been going for treatment.
Stanislaw says that around 9 p.m. that night, police came to the family’s home and told them the grave news.
“I didn’t sleep all night. I just cried half of the night. I didn’t eat nothing,” his father said. “I’m old, you know… if you’re young, you take everything differently. What am I supposed to do… to die?”
Police say Williams faces additional charges, including robbery and criminal possession of a weapon.
The NYPD could not provide additional information regarding the robbery charge.