My take on it is somewhat similar with the difference being that since the scene involved invoking a Mosaic law we should look that up and there we find that the 'justification' for capital punishment is that 'lest' the rest of the (presumably holy) community gets infected as in "one bad apple" process. That would make writing on the sand an analog for the /futility/ of keeping /records/ of judgment for an un-Holy people who are (spiritually) as sands in a dune subject to careless winds of material existence.