You are right in thinking that Georg Znaeym is the antagonist in this classic Saki story. We know before we are introduced to Georg that he is the implacable enemy of Ulrich von Gradwitz. Georg is introduced as:
The neighbour feud had grown into a personal one since Ulrich had come to be head of his family; if there was a man in the world whom he detested and wished ill to, it was Georg Znaeym, the inheritor of the quarrel and the tireless game snatcher and raider of the disputed border forest.
Enemies since "childhood", both Ulrich and Georg are therefore resolutely opposed to each other and harbour deep enmity. However, it is clear that in spite of this hatred, they are both men of honour, for when they finally meet they do not immediately give reign to their baser instincts:
Each had a rifle in his hand, each had hate in his heart and murder uppermost in his mind. The chance had come to give full play to the passions of a lifetime. But a man who has been brought up under the code of a restraining civilization cannot easily nerve himself to shoot down his neighbour in cold blood and without a word spoken, except for an offence against his hearth and honour.
Even though they hate each other, both are honourable men and will not kill the other in cold blood.
However, when the tree falls and traps them both, in spite of the initial threats they exchange, the pain that they suffer seems to dull their enmity. They swap their rancour for friendship, and interestingly it is Georg that imagines the scene of him and Ulrich riding into the market square together. However, in a twist of bitter irony, their shouts only summon a pack of wolves, who will no doubt prey on the trapped men and end all dreams of their reconciliation and peace.
Georg therefore as the antagonist starts of opposed to Ulrich, but what befalls them both gives them a new perspective on life and they plan their reconciliation, only to be stopped by the arrival of a pack of wolves. Befitting their enmity, they fall together and will die together.