Is there a protagonist and/or antagonist in "The Interlopers"?

There are, indeed, protagonists and antagonists in Saki's "The Interlopers," although these roles change during the narrative.
In the exposition and rising action of the plot of "The Interlopers," as the defender of his land, Ulrich von Gradwitz, can be considered the protagonist, who tries to protect the ownership of his land while Georg Znaeym,

...the inheritor of the quarrel and the tireless game-snatcher and raider of the disputed border-forest,

is the antagonist. When they come around the great beech tree, they face off against one another, but cannot immediately fire their guns because of their being civilized. Then, when they do think about killing each other, nature intervenes as the tree's large branches break from a deadly strike of lightning and the men are pinned under these branches.As they lie prisoners of nature's violence, the old enemies exchange words of animosity, each man hoping that his men will arrive first to wreak vengeance upon the other. But, gradually, Ulrich reconsiders his values, and he realizes that his animosity for Georg has been unreasonable. So, he offers his old enemy his flask of wine, and asks him to be his friend. Ulrich ponders this proposal for a while, then he agrees. Thus, they become friends:

And each prayed a private prayer that his men might be the first to arrive, so that he might be the first to show honourable attention to the enemy that had become a friend.

As they await help, both men now become protagonists because new antagonists near them. For, unfortunately, they have beckoned a new enemy; the men's cries have hearkened a pack of wolves.