
The environment they live in though (the Royals) is extremely competitive to the extent they all live in fear of being killed, or have no major issues with killing their family members in order to rise in status. The commoners too try to get some sort of status by exploiting/killing others if they are deemed the "other" or weak. Such is systematic in the plot so you can't really single out one character for being paranoid/exploitative. Prince Vichid it can be assumed is being poisoned by those he is meant to have trust in for example.
Also, I've forgotten exactly what Vichid did to Iain that was cruel. I know the other Prince talks about Vichid "keeping a man" and he was shown walking around with Iain, then in other chapters when he is being poisoned the latter is caring for the prince at his bedside. Guess they are both at this stage quite cold but also needy towards each other, but apparently something more important happened somewhere down the line. If you can recall what chapter(s) things went down?

http://www.mangago.zone/read-manga/trace_of_witch/uu/trace_of_witch-chapter-6/4/
Vichid raped him. Not for the first time either, apparently. And Vichid is shown to have always been sick, even as a child. He is a case of "if I'm hurting, everyone else must hurt too". He was poisoned when he got older, either because he kept ordering to kill all his guards and doctors or because of the power dispute.
I get why the characters act the way they do. It's a matter of survival, power hunger and a culture that makes cruelty something normal. But I believe that the point of Ian's character is to show that in spite of all that, it's still possible to be a good person. It makes the others seem even worse.
The scene in the last chapter, with the witches being tortured, it makes me wonder if there is anyone in the royal family who could possibly have ordered that. The first prince is dying, the second is dead, the third obsessed with his warlock and the forth seems too righteous for that. None of them would win anything by torturing them. It was probably the servants themselves, who were afraid and angry due to the recent disappearances in Uren. Does it make it ok? No. There is a huge difference between simply executing someone and torturing them for who knows how long, even if it's a execution via burning them alive.

Thank you. I agree that the servants probably take the decision to torture the prisoners (rather than it beind ordered by the royals/court). It gives them a chance to dominate and be superior which would otherwise be limited due to their low status and in their world power is gained and maintained through fear and violence. There was a film made by a holocaust survivor shortly after wwii which centred around the prisoners and the prisoners who were "elevated" to guard positions. Showing that the prisoner "guards" actually were more authoritarian and violent than thrme soldiers. So despite all being prisoners and sharing the same rooms in certain environments such attitudes do occur. Just as the servants try to maintain their own lives and gain a sense of superiority by 'othering" their fellow non regal prisoners and taking it on themselves to be cruel to them.

After reading those chapters , I agree that Iain seems the only character that doesnt appear to have been corrupted. Showing that even in such an evironment one does not have to retaliate with the same greed and abuse. He has little choice but to be Vichids servant and you would expect him to be angry at being treated in such a way.
Holly shit! That was disturbing as fuck! What the hell is wrong with these people?! It seems to me like Ian is the only good and sane human being in this castle. Everyone else has serious mental issues. O.o
I don't think our warlock is in any condition to help anyone right now, but Ian should try and ask his help to warn the first prince. I don't like Vichid, he is a monster and should die already, but before that he could at least innocent the guy whose life he made a living hell. I hope Ian manages to escape somehow, he deserves better. And I'm 100% on the warlock's side and really want things to go well for him, but I highly doubt this story will have a happy ending for anyone. ('~')