Wilson is an English hunting guide (a professional, which is to say he does not let emotion or opinion get in the way of his job), and he always gets his prey. He knows that his clients are along for adventure that they have partly made up in their heads, and he is willing to go along with their fantasy. To a point.
In many ways, Wilson is the exact opposite of the hen-pecked Macomber. Wilson's "flat, blue, machine-gunner's eyes" (1.68), as Margot sees them, are both cruel and seductive at the same time. When we meet him, he is wearing "old slacks," "very dirty boots" and a necklace of ammunition. He is rugged, handsome, and ready for the hunt.
He has courage in spades, too, and doesn't shy away from the violence of the kill. You won't see this guy running from a lion on the charge. The guy has been through a war. In fact, his worldliness and experience stand in contrast to Macomber's narrow life as a wealthy American. Having fought in World War I, Wilson has seen boys become men before, unlike our friend Macomber, who has only just now become a man.
Wilson doesn't expect much from people either. He's satisfied with free whisky, his fee, and the occasional client's wife. He wants to keep his distance because he wants to avoid going through "this emotional trash," which, of course, the Macombers are lumping on him every chance they get. Still, he manages to remain relatively detached, which puts him in a unique position to make some keen, colorful observations about the Macombers. He writes Margot off as a cruel, but attractive (and therefore typically American) woman, and her husband is a weak coward.
But just how much are we supposed to believe in these judgments? How much are we supposed to relate to Wilson? In many ways, he is the character we are closest to; we enter his thoughts quite a bit. But he sleeps with his clients' wives, drinks their whisky, and at the end, seems willing to cover up a murder. He might not be the most moral man ever, so when we watch him watching the Macombers, we might want to take his observations with a grain or two of salt.