VIGGO CANONICALLY CARED ABOUT DRAGONS (to an extent) EVEN IN THE EARLIER SEASONS.
Dagur said he learned how to make the cure to dragon root poisoning while working with Viggo and the hunter's.
Which means that Viggo had not only gone through the effort of finding but also actually making a cure to make the dragons he had captured feel more comfortable. Or, at the very least, less sick.
This, combined with Viggo's relaxed demeanor towards Grump, shows at least *some* level of care or appreciation for dragons. Maybe it was from a business perspective of wanting good quality/not sick product to sell. But even then, he went through the extra effort. He didn't have to, but he did.
Hell, maybe that's why it was even possible for him to change in the first place. Aside from Hiccup's influence and being saved by a dragon, Viggo already had a certain amount of respect and appreciation for dragons. He just needed a little extra push.