Answer:
The speaker realizes that the old man's eyes are open, which means that the old man would be able to tell everybody who attacked him.
Step-by-step explanation:
The eyes of the old man are open in the scene. This is what the speaker notices about him, and disregards everthing else according to the statement "...I could see nothing else of the old man's face or person".
The fact that the old man's eyes alone were open meant a threat to the speaker's interests, and so the speaker proceeds to murder him.