When contemplating the murder, the narrator's greatest fear is?

The narrator's greatest fear is delay.... not being able to go through with the murder.

Tomorrow arrives, and with it a more impatient anxiety to do our duty, but with this very increase of anxiety arrives, also, a nameless, a positively fearful, because unfathomable, craving for delay. This craving gathers strength as the moments fly. The last hour for action is at hand. We tremble with the violence of the conflict within us,—of the definite with the indefinite—of the substance with the shadow. But, if the contest have proceeded thus far, it is the shadow which prevails,—we struggle in vain.