Why does it make a game-changing good impression on the Melkotian that Kirk won't kill what everybody knows is a fake person with what everyone knows is a fake gun?
But do they know they're fake persons? Spock does indeed come to the conclusion that this is an illusion of some kind, but after all if the landing party are all real, the locals might also be real. And if Chekov could be killed(and he isn't sure that he isn't dead at this point), so could they.