How does Dunstan justify to himself stealing Silas' gold?

In his own mind, Dunstan convinced himself that Silas was dead, and as a result, no one would ever know.

Dunstan's own recent difficulty in making his way suggested to him that the weaver had perhaps gone outside his cottage to fetch in fuel, or for some such brief purpose, and had slipped into the Stone-pit. That was an interesting idea to Dunstan, carrying consequences of entire novelty. If the weaver was dead, who had a right to his money? Who would know where his money was hidden? Who would know that anybody had come to take it away? He went no farther into the subtleties of evidence: the pressing question, "Where is the money?" now took such entire possession of him as to make him quite forget that the weaver's death was not a certainty.