Why is Ulrich not able to shoot his enemy when he first encounters him in "The Interlopers" by Saki?

Ulrich von Gradwitz is unable to shoot his enemy when he suddenly encounters him because of the "code of a restraining civilization." This code prohibits shooting a neighbor in cold blood without warning or words exchanged unless the offense is one against a man's honor or his family.

The feud that exists between Ulrich von Gradwitz and Georg Znaeym is simply over a particular strip of very steep woodland on the edge of the Gradwitz...

Ulrich von Gradwitz is unable to shoot his enemy when he suddenly encounters him because of the "code of a restraining civilization." This code prohibits shooting a neighbor in cold blood without warning or words exchanged unless the offense is one against a man's honor or his family.

The feud that exists between Ulrich von Gradwitz and Georg Znaeym is simply over a particular strip of very steep woodland on the edge of the Gradwitz forest lands. It is jealously guarded by von Gradwitz because of the fact that it was wrested from the Znaeyms in a famous lawsuit. However, although the Znaeyms lost in the courts, they continue to come on the land and poach, and the feud is continued by the impassioned younger male members of the families. 

One night, as the deer who normally are bedded down are running during a storm wind, von Gradwitz calls upon his foresters to go out and search for any sight or sound of "the marauders." It is on this "wild night," then, that von Gradwitz suddenly is confronted by his mortal enemy, Georg Znaeym. But, in that short moment of civilized conditioning in which they cannot shoot each other, the mighty beech tree around which they are face-to-face is struck by the violence of nature. Suddenly, a mass of branches crashes down upon the two men. Now, helplessly pinioned beneath the broken branches and wounded, the two enemies clearly cannot end their feud as intended.