The narrator believes that his audience will approve of and even laugh at his plot to murder the old man.
The fact that the narrator believes the audience will approve of his actions to murder the old man shows that he is unreliable. The other options are things that a sane person could do and be seen as compassionate. It isn't until the narrator shows his obsession with the old man and the desire to murder him that the reader becomes suspicious of the mental state of the narrator and his reliability.
