Kastil, faced with the evidence he has in the order he receives it, thinks Caris is probably an enemy to the Empire, and that the most likely version of that is that he serves Asmodeus.
Altarrin didn't think so. And the thing is, if Caris made a mistake on this level, that implies that his mind-control abilities, if they exist at all and were successfully being concealed - which the Emperor cannot in fact rule out, on further introspection it is suspicious just how quickly he fell for Caris - are probably limited.
(Bastran is not, currently, thinking of Caris as a lover. This is absolutely not a situation where he should or can afford to be thinking with his dick – and he suspects Caris would judge him for it. Those feelings are boxed away; right now, he's reasoning about Caris as an otherwordly stranger with unknown capabilities, who convinced Altarrin to take his side.)
Maybe Caris was able to nudge Altarrin's impression of him, come across as more harmless and more - aligned with the Archmage-General's goals - then he really was, but Altarrin is very paranoid. If it were just that, would he really have committed so deeply just on the basis of a vague positive impression? It feels off. It feels very off that, on that basis, he would have flung himself through a Gate into a room where he would almost certainly die, just to buy Caris a few seconds of warning.
Kastil, of course, thinks this is further evidence for the mind-control hypothesis, and on the face of it, it does seem very very out of character. But is it? Or is there an explanation, a possible set of circumstances, for why the Altarrin he knew would sacrifice himself to get Caris out.
Bastran has always known that Altarrin would give his life for the Empire, if that was the trade he was offered. Not everyone would, but Altarrin is someone who would do it of his own will, even if his compulsions weren't forcing the matter.
Would he give his life, not just to protect the Empire from an existential threat, but to hold onto a valuable resource? ...Actually, yes, if it were valuable enough. If they're really in the world where Caris is mostly aligned with Altarrin's goals, and is their only hope for truly making headway against the gods.
But really the thing I'd do is a life after death, Caris had said to him. I hate it, that there isn't one. It feels like it's lurking over my shoulder, every day, every waking hour, that this is it, and it's so fragile... I'd figure it out.
It's not an incredibly convincing argument, certainly not one that will convince Kastil, to claim that Caris saying that is particularly strong evidence of Caris meaning it, even if Bastran is setting aside the strong mind-control theory and assuming weak mind control at best. Altarrin's people, when questioned, admitted that Caris was a very good actor, and skilled at controlling his thoughts while being read, and of course Kastil believes that Caris could have just made people see what he wanted them to see.
But to the Emperor's gut, it feels like it means something.