Answer:
Alec's arrival leads Judson to drink the poisoned whiskey and most probably to his death.
This is related to the theme of the story because Judson tried to get justice in his own hands, leading to him getting the same treatment.
Explanation:
The short story "Ruthless" by William DeMille revolves around the story of Judson Webb and his "trap" of putting poisoned whiskey for whoever broke into his cabin house while he's away. The story has a dramatic ending with that same whiskey being given to Mr. Webb.
Alec is the caretaker of the cabin while the city folks are away. So, after Mr. Webb put the rat poison in the whiskey, he was on his way to return to the city. But he slipped on an acorn, knocking him out for some time. And when he gained consciousness, he saw that he given whiskey, which seemed to be his poisoned whiskey. So, Alec's arrival leads to Judson drinking the poisoned whiskey.
This relates to the main theme of not taking things into one's own hands. Trying to exact justice into one's own hands would lead to serious consequences.