If I'm piecing the evidence together right, he heard the burglars break into his neighbor's home, called 911, and waited inside of his own home for the police while the burglars took their time to ransack the home. He got agitated believing they were going to get away scot free, grabbed his shotgun, and told the 911 operator he was going outside the confront them. When he stepped outside he found them just 15 feet away, coming toward him. He warned them, then fired. He hit one in the chest (corroborating his version of the facts, that they were approaching him) and one in the side, probably as he was turning away to run after the first burgalar got hit.
Under the circumstances it appears clearly to be a case of self-defense.
I definitely agree.
The only real mistake Horn made was broadcasting to the dispatcher that he intended to do something to stop the burglars.