he/she runs away with no shots fired. What do you do next? Call the police and run?
Scan the area for additional threats.
If no threats (check really carefully), reholster.
Scan for witnesses to the attack. If you find someone, engage him/her (after reholstering): "Hey, buddy. Wow, did you see that? I'm going to call the police." See if you can get the witness to stay and speak to the responding officer.
If the attacker presented/fired a gun--reasonable assumption: there was a reason I presented mine--understand that you might not be the only one calling 911, and the police are responding to a man with a gun/shots fired call. Consider asking someone in charge (say, the manager at McDonald's) to secure the door and to ask witnesses not to leave--the gunman might be waiting. If a gun was fired, see if someone is injured.
After you call 911, see if you can stay on the phone with the operator until the responding officer arrives. The 911 operator may have a script and directions for you to follow. Follow them. Make sure the officer knows what you look like, what you're wearing, etc.
If the operator has no script: As you see the officer approaching, make sure you are standing in a well lighted area: if nothing else is available, stand in his head-lights. Hands are up, away from you waist, clearly visible. Ask someone (witness/manager) to go to the officer, obey him, and lead him back to you.
If the officer asks you to remove your gun, do not touch it: lift your arms to the sky (slowly--everything slowly), and say, "The gun is yours, officer. I will not move. I will not touch it." (If the environment permits it, consider avoiding this contingency by unloading the gun, slide locked open, and placing it in front of you on the ground before the police arrive.)
Different officers/departments will have different ways to respond to these calls. None of them are willing to be shot. If shots have been fired, they will rightfully be edgy. Don't do anything that might make them fear for their safety in any way.
Oh, and if it was just you alone, no witnesses: ask the 911 person if you should stay there, or meet the officer somewhere. If the area is unsafe, say you can't stay, and find out where to meet the officer, and ask what to do once there. No misunderstandings.
Tactics has been defined as the art of not getting shot. That includes not getting shot by a well meaning police officer trying to survive her/his shift.