The victim was identified as 54-year-old Edward McMahon of Tujunga.
A Sunland man told police he heard noises outside, got a handgun and saw McMahon stealing gasoline from his car Monday morning. The homeowner said he ordered McMahon to leave and shot him when he refused.
McMahon died at the scene from a single gunshot to the chest.
The homeowner, whose name was withheld, was detained but later released pending further investigation.