Investigators discovered two notes Geddy Kramer wrote before he went on the shooting spree that injured six people April 29.
One was found inside his vehicle parked outside the FedEx facility, the other at his house in Acworth, Ga.
"I'm in my happy place," Kramer wrote in one of the notes. "I'm in my happy place. I'm in my happy place."
He said he felt isolated, did drugs and had mental issues. He said he planned to shoot anyone who sold him drugs.
"This wasn't the result of media brainwashing and subliminal messages," Kramer said of the planned massacre. "This is my own doing. I'm a sociopath. I want to hurt people."
"I'm not sad or sorry for the misery I've caused," he added.