This man is not a responsible gun owner or man who was defending his life (except at the very start). He acted out cold-blooded murder when the threat was already neutralized.
It is in the interest of gun rights and of basic human morality to condemn what he did. If what we know so far is true, he should never leave a cell again.
That said, innocent until proven guilty.
13 Pistols, 5 Rifles, 1 Shotgun, no time to shoot them