From this story you don't know many things. Was this a street with houses? Apartment complexes? Were they on the property or nearby? We have no basis to know if the nephew saw the event or related stuff he heard etc..
All that said, if the man came out and told the police to GET OFF HIS PROPERTY and they didn't then the cops are liable here. They can tell him to put down his gun but without proper ID then why should he?
This story lacks certain facts.
I agree the information is sketchy, but it seems clear to me that the old man didn't know or didn't trust who they were, which was the heart of what led to the shooting. That could have been alleviated by the department letting him know ahead of time that they would be there.