If I understand the charge correctly, that's not the way it works. It's a death ancillary/subordinate to the commission of a crime.
The crime being committed was assault with a deadly weapon. Pointing a gun at someone is assault. They pointed guns at him before he tried to wrestle a gun from them. Just because the men thought they were performing a citizens arrest doesn't give them the right to confront someone with firearms.
I think you may be mistaken in that - Georgia Code. Under Georgia law murder requires malice, expressed or implied. Murder committed in the commission of another felony doesn't require malice.