It looked odd, using the big gutting knife to clean his nails. I guess he did it to put a scare into the man, scare him without threatening outright.
Speers is smart like that, doing just enough that you get your imagination working against you. He told me that a man is always his own worst enemy, that if you can say or do the right thing you can break him easy as a twig.
Maybe it was the knife, or the fact Speers and me had him tied up in the jonboat. Or maybe it was the swamp itself.