It's probably worth elaborating (if only for the sake of getting my thoughts straight) on why Toothless took off in a huff from a conversation with Merlin and yet stayed and chatted to Arthur for a long time in their post the other day, despite the fact that Merlin is an adorable derpy dragonlord who loves everything and Arthur is Hater McGenocide Jr. with regards to dragons. He gets on better with Arthur most days than he does with Merlin! I've decided this is because Arthur is a prat and really, Toothless is too. But there are other reasons!
Toothless has gotten a lot more used to hanging out with humans who aren't Hiccup in camp. He likes most of the ones he's met, sometimes a lot! Super sweet and adorable characters who are very kind to him, like Vanya, do have the power to top him mercilessly. He's a pretty straightforward tsundere type! Except when you touch on his issues with humanity, which are still present if in the background of his life these days. Merlin managed to do that by insisting that Toothless agree to give Arthur a chance.
To Toothless's mind, he is more than giving Arthur a chance already. Arthur THREATENED him the first time they met, and Hiccup and England had to intercede. Yet he's still willing to talk to Arthur! He'll answer Arthur's questions to the best of his ability! Toothless is doing this because it's interesting to poke back at Arthur for his reactions, and because he kind of likes Arthur's style, though he wouldn't admit that last part to himself. But Toothless is very certain that Arthur didn't earn that from him, not the way Hiccup did. He's doing it because he wants to, and if something changed, he'd change his mind about it. To have Merlin ask that kind of tolerance as a favor when Merlin hasn't earned it either was enough to piss him off, especially because he thinks it's hypocritical when Merlin is lying to Arthur as it is.
Toothless is uninterested in hurting humans, because it'd hurt Hiccup, and because he has a new discomfort with the idea on his own because of close association with humans for so long now. But he's very clear for his own part that this is a choice he made. He is a Night Fury! The unholy offspring of lightning and death itself! Even grounded without Hiccup's help, he's totally capable of killing a normal human if he wanted to with very little effort, and he's proud of that offensive capability. When he's dealing with humans, he wants them to recognize both that he should be respected as something dangerous, and roll with the fact that he has decided not to be. He gets pissy if people get too scared of him or act as if he's no threat and not worth respecting. (He totally makes
this face in the movie when Hiccup is telling Astrid that "there's nothing to be afraid of.") This may not be entirely fair but he's okay with that. He's made a lot of changes for you crazy humans! How dare people ask him for more concessions than he's given already! YOU SUCK I AM LEAVING! Etc.
In conclusion: the dragon is kind of a dick, but we knew that. Angrydragon with a heart of goooold. Love me 8(