Looks like that was the justification;
"In the moments before pulling his weapon and firing, Gerlach said he saw what he thought was a gun in Kaluza-Grahams right hand. He said he could only see silhouettes, but was afraid Kaluza-Graham was pointing a pistol at him through his back window.
I thought, This is it, Gerlach said."
Thanks for pointing that out.