Why does Muff Potter return to the scene of the crime?

Muff went back to the scene of the murder in order to retrieve his knife.

"Is that your knife?" and it was thrust before him by the Sheriff.

Potter would have fallen if they had not caught him and eased him to the ground. Then he said:

"Something told me 't if I didn't come back and get—" He shuddered; then waved his nerveless hand with a vanquished gesture and said, "Tell 'em, Joe, tell 'em—it ain't any use any more."