Pragmatic Villainy: Dagur claims that unlike Drago, Viggo is a rationalist with a business to run. Even if he did know about kids with magical powers, he would recognize it was way to unpredictable for his troubles and would steer clear of them. The fact that the triplets have an entire family of magical dragon riders looking for them only proves Dagur's point.
Dagur: If he ever found out that magic was real, he probably wouldn't mess with it. He'd think about it like this. Either he screws around with supernatural forces he doesn't understand, and it comes back to bite him in the butt. Or he leaves it alone, and keeps selling his dragons. Even if he did run into those kids, the moment he found out they were magic, he'd leave them right where he found them. It wouldn't be worth the risk, to him. And he definitely wouldn't sell them. There's a chance they could fall into the hands of one of his enemies, and he can't have that either. I'm telling you, Viggo doesn't have them.
Unstoppable Rage: When Hiccup points out to Dagur that he has no one to blame but himself for his life's downward spiral, an enraged Dagur breaks out of his restraints and nearly strangles him to death.
