Because in the novel, those fake regas has hidden ulterior motives driven by human desires and Karas know it. So before dinner, Karas instructed all fake regas who will be present to apply the scented oil he has given on their body.
So, by the time Truyde introduced them to Karas, Karas burned them without second thought. I feel pity for them too but can't be help since they sided with Truyde. They died without even screaming and turned ash. Of all the decendants of the Dragon King, Karas was the first and only Dragon King who has the same instinct as the True Dragon.
And yes, Karas is soooo smart that all that has been happening is boring for him, even the game with Truyde is also boring for him that he decided to destroy the world if it weren't for Abel tho.
So, Karas can't be reason with as a human being, but as a beast and dragon. I wouldn't be surprise why he did that AHEM to Abel. (▰˘◡˘▰)
Karas, what the fuck did those random blond twinks do to you? What's even the point of doing that? Just keep setting the council members on fire you idiot. The regas are just a commodity to them, so it's not like it's going to actually upset them to have some die. Please God just actually focus on doing something about Duke Truyde who's an actual credible threat to you. Maybe I'm just too much of a simpleton to understand his big brain plan, though, so who knows.