Expedition: general discussions.

the first is a demon of sloth, and even if it's in exchange for a favor, it is summoned to help us.

He simply doesn't care about anything because he is a demon of sloth.

but even if the player falls for the trick – reveals himself in the final part of the prologue and gives useful advice for the future, instead of turning the situation to his advantage.

It's just protection against IDIOTS who click on dialogue options without reading the text; there's no point in it.
And you yourself wrote that, regardless of the circumstances, demons always embody evil (no matter how they got there).