"Nichols told police Wilhelm identified himself as a federal agent, which prompted him to kill the man."
Nichols should now be in Federal custody, awaiting trial in a Federal Court, found guilty and executed in a Federal prison. As it is, his trail will be a circus act in an Atlanta court room where he will be tried by a jury of his peers, and we will in all likelihood see this man beat the death penalty.
I think he'd be more likely to be executed if prosecuted under Georgia law than federal law. I'm no expert, but I was under the impression that it is harder to execute under federal law.