The best way to summarize the narrator's internal conflict is
that he is
O
indecisive about killing the old man.
confused by the odd quiet in the house.
afraid he will lose his nerve and run away.
worried his own heart will explode from terror.
O

indecisive about killing the old man.

Explanation:

The other three options like "confused by the odd quiet in the house.", "afraid he will lose his nerve and run away.", "worried his own heart will explode from terror.", the selected sentence says about his inability to take a decision which is a internal conflict that the subject was facing in the given sentence. Also, the sentence, "indecisive about killing the old man." is a great moral conflict the subject has to choose between being good and bad.