KPTV reports that Christopher Dillingham changed his plea Thursday.
Court documents say the man told officers he killed the dog last August because he believed his ex-girlfriend had "put the devil in it." Officers responded after the blast alarmed neighbors.
He reportedly also said he was preparing for "the rapture" and added that "the world is going to end" due to a nuclear strike.
Investigators believe Dillingham used black powder from fireworks to make the bomb that killed the animal.
He was sentenced to about a year in jail, with credit for time already served.