You are not required to speak with any police officer or provide any information. You may also be able to file a complaint against the officer for his conduct. You should consult with a lawyer to discuss your rights. I'd be happy to speak with you. Please call me at 404-812-4305 if you would like to discuss this matter.
It appears from your post that the police department is investigating a serious crime (aggravated assault, and/or homicide). Given the seriousness of the crime, it is not uncommon for law enforcement officials to canvas potential witnesses and to try to persuade them to cooperate in the investigation. With that said, as you have already provided law enforcement with your account of what you saw, you do not have a duty to continue to cooperate with them absent a grand jury subpoena or other order from the court. With regard to your assertions that a member of law enforcement threatened you, you have several options. (1) You can leave it alone and keep the tape in case the harassment continues; (2) You can go to Internal Affairs (OPS - Office of Professional Standards in Atlanta PD) and file a formal complaint; (3) You can report the incident to the District Attorney handling the case and ask that law enforcement refrain from further harassing you. If 1-3 do not work, you can contact an attorney to help you.
I agree with the answers previously given, but I also want to recommend one thing further. If this was Atlanta Police Department (APD), there is a citizen oversight board created to investigate complaints independent of the police department. While they are undergoing changes to membership and structure at the moment, filing a complaint with them will still be the most effective method. The Atlanta Citizen Review Board (ACRB)'s website is: http://acrbgov.org/
You should file a complaint so that these actions do not continue to go unchecked. Good luck.