The reason Viggo kidnapped you was simple - rumors.

Of course, at first, he did not believe in the stories of a viking that possessed the ability to tame even the most rabid Monstrous Nightmare with their mere presence. But when his trusted intel informed him that Hiccup and the Dragonriders had started looking for you, Viggo knew that he couldn't take any chances -- if the stories were true, you could be his greatest weapon in defeating the Dragonriders and dragonkind.

You were being dragged to an arena, previously used to hold dragon-fights, judging by the various burn- and clawmarks across the floor and walls. The holes between the gates leading inside provided you with a view on what was awaiting you, and it was real bad. A mean looking Skrill lashed around, its wounded body proving that it had been tortured to show an unrestrainable level of aggression.

"We will see if those stories are true", Viggo said coldly, motioning to his men. A moment later, you were thrown in with the beast.