I think Reinhardt was just tweaking the other two judges on the panel.
Based on a speed read:
The defendant Dickens was convicted of murder and sentenced to death in Arizona state court for his participation in a robbery and murder. He didn't pull the trigger but was considered a "major participant" which is a prerequisite for the death penalty. This was based, in part, on the fact he was aware of the robbery, drove the car to the store, and either was aware of or provided the gun to the triggerman. He also knew the triggerman had beaten up a nurse, often carried guns, twice pointed a gun at defendant Dickens' head, and bragged about other murders. The state court noted this helped prove Dickens' reckless indifference to human life when he helped with the robbery. This is the context in which Reinhardt referred to the Second Amendment and the fact a large number of people in some states carry guns. It is as if he is lumping all those who carry in with the piece of trash who pulled the trigger.
The federal courts review the state's findings and application of constitutional law under a very highly deferential standard which the majority applied here. Reinhardt is legendary for writing habeas opinions which the Supreme Court reverses.