When Dunstan takes Wildfire to sell it, wildfire is killed and Dunstan decides to rob Silas.
Dunstan is a selfish brat. He takes the horse Wildfire to sell because the family needs money. He arranges to sell the horse to Bryce and Keating, who happen to be hunting. Dunstan does not sell the horse to them right away though. He rides the horse, and decides to take it jumping fences. Unfortunately, the horse is killed when he jumped a fence and didn’t make it.
Dunstan, however, took one fence too many, and got his horse pierced with a hedge-stake. His own ill-favoured person, which was quite unmarketable, escaped without injury; but poor Wildfire, unconscious of his price, turned on his flank and painfully panted his last. (Ch. 4)
Dunstan got away fairly without injury, but that is not the end of the story. Actually, in some ways it is the beginning of the story, because he decided to go and rob Silas Marner. Silas, a man on his own who never hurt anyone and never bothered anyone, was an easy target. Dunstan, a spoiled brat, found him there in his hut alone in the countryside and decided to go for it.
The idea of Marner's money kept growing in vividness, now the want of it had become immediate… (Ch. 4)
So Dunstan, who never thought of anyone but himself, decided to go to the cabin. He thinks that if Silas Marner is dead, and he has his money stashed there, he can find the money. If not, he worries that he might and find him when he is looking for it. Dunstan, “whose nature it was to care more for immediate annoyances than for remote consequences,” will go ahead and steal the money anyway.
It may be ironic that Godfrey and Dunstan Cass have money and Silas does not, and Dunstan is the one who robs Silas, but it is true. Dunstan is no good. He is morally corrupt. Yet the Cass family and Silas are not done interconnecting. Silas is a recluse now, but soon he will be forever entwined with the Cass family through Eppie.