Dunstan is a bad, bad boy—and not the sexy kind. He's just bad. He's a liar, a blackmailer, and a thief, and we breathed a sigh of relief when he turned up dead.
So why is Dunstan this way? The narrator doesn't explain, but it probably has at least a little to do with his father, Squire Cass, and the fact that his mother died when he was young. Dunstan, also called Dunsey, is "a spiteful jeering fellow, who seemed to enjoy his drink the more when other people went dry" (1.3.3), and he takes a "delight in lying" that is "grandly independent of utility" (1.4.4)—that is, he lies for the sake of lying.
Let's take a look at Dunstan's rap sheet: tricked his brother into a disgraceful marriage; stole the rent money; blackmailed his brother into selling his horse; made off with that horse and killed it by taking a stupid risk; left the horse on the side of the road; and then rounded it off by stealing a poor old man's stash—not to mention whatever "wild oats" he's been sowing (1.3.3). Impressive for a guy who's only in his early twenties.
Well, we know that Dustan's mother died young. Considering what a big deal the narrator makes about Dolly Winthrop's mothering skills, this dead mom does seem to be a major problem. "The Red House," we learned, "was without that presence of the wife and mother which is the fountain of wholesome love and fear in parlour and kitchen; and this helped to account […] perhaps, also, for the fact that his sons had turned out rather ill" (1.3.3).
Another problem is that Squire Cass isn't much of a father. "He had kept all his sons at home in idleness," the narrator says, meaning that he didn't send them to school or out to learn a profession. With nothing to do, Dustan practically has no choice but to turn evil.
Without knowing more about Dustan, and we really don't learn too much about his motivations, the best way to understand his character might be to think of him as a sort of Iago-figure, somebody who likes to make mischief just for the sake of mischief. He doesn't like other people to be better than he is, and he drags everyone down with him. But would Silas have been redeemed without him? If Silas still had his money, would he have taken in Eppie?Dunstan Cass's Timeline