In chapter 4, Wildfire is supposed to be sold. Dunstan knows that his brother Godfrey is secretly married, so he tries to blackmail him. As a result, Godfrey agrees to let Dunstan sell Wildfire.
It ended in the purchase of the horse by Bryce for a hundred and twenty, to be paid on the delivery of Wildfire, safe and sound, at the Batherley stables. (ch 4, p. 19)
Dunstan knows that he should hire a horse to take him back, but instead he decides to run Wildfire. Unfortunately, he “took one fence too many” and the horse was killed. Dunstan, who is not worth anything, is not hurt at all. The more valuable horse is a goner.
Rather than feeling bad about the horse being killed, Dunstan is just glad no one saw him. Dunstan's nature is "to care more for immediate annoyances than for remote consequences" (ch 4, p. 19). He picks himself up and walks.
Dunstan then goes off and enters Silas Marner’s cottage, surprised to see that the man is not there. Dunsey thus helps himself to Marner’s gold.
This incident of course demonstrates how selfish and conceited Dunstan is.