What should he have done to avoid prosecution?
Rolfe and his partner should have apprehended Garrett using non-lethal force, or else just let him get away.
[Deadly] force may not be used unless necessary to prevent the escape and the officer has probable cause to believe that the suspect poses a significant threat of death or serious physical injury to the officer or others.
A taser, as various police forces have assured us, does not pose a significant threat of death or serious physical injury:
In its report, the panel said that while CED use is not risk free, there is no clear medical evidence that shows a high risk of serious injury or death from the direct effects of CEDs. Field experience with CED use shows that exposure is usually safe.
Since Garrett was running away when he was shot, and did not take any time to aim the taser, there is no reason to suppose that he would have attacked Rolfe or anyone else if he had succeeded in hitting him. Also his partner was present and would certainly have intervened.
In addition, at the time when Rolfe shot Garrett the taser had been discharged and therefore presented no danger to anyone.
Hence there is a strong case that Rolfe was not justified in using deadly force against Garrett. If deadly force was not justified then its use is a crime.
In Tennessee v. Garner, 471 U.S. 1 (1985), the Supreme Court held that, under the Fourth Amendment, when a law enforcement officer is pursuing a fleeing suspect, the officer may not use deadly force to prevent escape unless "the officer has probable cause to believe that the suspect poses a significant threat of death or serious physical injury to the officer or others."
A fleeing suspect who has already used up the capacity of the Taser and who has no other weapon does not pose a significant threat to a reasonable officer.
The officer could have not shot the suspect in the back, could have refrained from kicking him when he was down and could have arranged immediate medical help. News reports say the officer broke seven use of force rules of the Atlanta police force.