If the guy is still standing with a weapon in his hand, then he remains a legitimate target for another round.
If he's on the ground, but still has a handgun in his hand and is still conscious enough to potentially raise his arm and fire, then he remains a legitimate target for another round.
If someone assumes the prone position, that could be taken as the most accurate position for shooting at an innocent person. If they are still a threat, you have a legal right to defend yourself in almost all jurisdictions. The trick is talking with a lawyer before talking with anyone else, and then properly and accurately describing the events to defend your actions.