That sure is impressively horrible. Creatively horrible. Like someone put a lot of effort into coming up with the most horrible thought experiment they could. And was better at it than Seldan, because he would not have come up with that!
Those are his first thoughts, but he isn't exactly feeling horror. It doesn't seem like it would help, and Seldan may not have been capable of inventing that but he was, in his time, an acknowledged champion of horrible thought experiments. He can look through the memory with detachment. Mostly. Except for a brief moment of idly picturing kicking in Vicar Rey's head, the thought tagged with an acknowledgement that just because someone did awful things does not make it fine to do whatever you want to them and he wouldn't actually go enact a revenge fantasy about it, but also he absolutely won the argument once about whether having revenge fantasies you're never going to act on is fine actually.
It's quite clearly demarcated in his thoughts that the horror isn't toward Blai. He wants to understand Blai, and this is a step toward that. There's some admirable tenacity and self-control to show there.
He can guess it gets worse than this, in terms of things Blai has actually done that were horrifying (which "get kicked down the stairs and locked in a basement to coerce him into praying to an evil god" isn't); all he's sure of is that it didn't involve eating babies. But, well, if this was just a horrifying thought experiment, rather than Blai's real actual life, it would be something about exploring the question of when a person's actions in extreme circumstances start to come apart from their character, in the sense of what you could predict about their actions in normal circumstances, and how this relates to moral culpability. One can take a position at either end on that question, and Seldan has always thought of himself as somewhere in the middle. It's information about what sort of person Blai is, and he's glad to have that information; he wants to understand Blai. It's already pretty clear that it's not predictive of Blai's character, in the sense of how he behaves when offered magic by a god who isn't evil (who sounds pretty excellent, really) and then dropped in a new world and given an opening to save a kid and prevent a war.
He is also no longer confused about why Blai doesn't really like being referred to as Seldan's Chosen! That makes sense!
Oh. Also. If anyone else ever again tries to do that to Blai, he will kick in their head. He feels very strongly about that. NO ONE is allowed to try to mess with Blai anymore.
(Thoughts he's not having where Blai can see them: any expression of sympathy on how unpleasant and awful that sounds, he doesn't think that will help. Getting competitive on horrible-thought-experiment-generation and trying to see if he can think of an even worse version. Ranking all the Heralds he's met so far by how likely it is they could end up being chosen by Asmodeus if plopped into the horrible thought experiment.)
trying to drink a candle is a little funny though
like, in an awful tragic way, but still