NEW YORK — At least 400 people are killed by police officers in the United States every year, and while the circumstances of each case are different, one thing remains constant: In only a handful of instances do grand juries issue an indictment, concluding that the officer should face criminal charges. Successful prosecutions generally involve officers who have lied about what they've done, tried to cover up their actions, or used excessive force to inflict punishment. Even as protesters took to the streets Wednesday to decry the failure of a grand jury to indict an officer who used a fatal chokehold on an unarmed man in New York City, a grand jury in South Carolina voted to bring murder charges against Richard Combs, a small town police chief who fatally shot an unarmed man who had come to Town Hall to contest a traffic ticket.