Well yeah....and this defector could well end up as another pretext.
But I sort of get the impression that the real reason for shooting in this case is that this happens so rarely that the officers just did not know how to react....confusion, pure and simple.
probably true
One problem is that an armed South Korean national showing up in NK could not only get shot for his trouble, but be used as an excuse for war.
“South Korea just invaded us!”
...and technically, they’d be correct. You can’t let armed South Koreans cross the border into NK.
Of course, at a distance, you might not be able to discern if he was armed or not. Too bad for him...on the DMZ you have to shoot him to be sure.