The issue is that police are allowed to use lethal force when they are in jeopardy of serious bodily harm or death. Any citizen in Georgia is allowed to do the same as long as it meets the ocga 16-3-21 code section. That DA caused APD to lose all mutual aid and crippled morale by incorrect interpretations of the law. The officers are probably concerned that they will be criminally charged if they get in an OIS even if it's justified. There is an inherent risk of being involved in an OIS if you do the job so why would you stay if the DA will

Regardless of your feelings towards the incident, legally it was justified. Anyone in the state is permitted to use lethal force when they are being are in reasonable fear of serious bodily harm or death and it's in defense of themselves or a third party (ocga 16-3-21).

The DA charged other officers with aggravated assault for using a taser on those college kids so he knows the device has a potential for serious bodily injury or death (this is why it's called less lethal opposed to no lethal). He established that the use of a taser meets the requirements for agg assault (ocga 16-5-21), so the officer would be covered by 16-3-21.