I don't believe the reason why what they did wrong was properly articulated.
They did not have justification for an arrest. The law is very specific.
2010 Georgia Code :: TITLE 17 - CRIMINAL PROCEDURE :: CHAPTER 4 - ARREST OF PERSONS :: ARTICLE 4 - ARREST BY PRIVATE PERSONS :: § 17-4-60 - Grounds for arrest
By their own words the duo was stopping the man "to speak with him". This is not justification to make a citizen's arrest. If there is not a justification for the citizen arrest, everything they did becomes a crime - period.
The way they carried their weapons is brandishing. The attempt to stop and hold the man is attempted kidnapping. Since kidnapping is a felony, in most states, this makes the death which occurs afterwards, murder - regardless of who is the cause. Now Georgia may be ok with criminals claiming self-defense when they shoot the convenience store clerk, but usually involuntary manslaughter is still charged.
So once again, it doesn't matter what happened during or just before the shooting. All that matters is if the duo can prove the arrest was legal. Now what do you think is that chance of that is with a non-racist jury? Somewhere between nil and zero.
What happened afterwards is irrelevant. The duo created the situation which caused the death. The duo is responsible for that death.
Now, I will tell you what will happen. This duo will end up dead or jailed for a long time. They might have gotten away with a short or suspended prison sentence before their stupid lawyer decided to make it national, but now they will not be getting off easy.