Admitting one's weakness to a demon is an elementary error at best. Perhaps standard fatigue is all they may blame, though they're certainly inclined to believe that Shiro is stricken with a fatal case of baseless trust.

"No rest for the wicked?" Their tone is not soft. It is, perhaps, not as hard as it has the potential to be. To cultivate a false sense of security, possibly. That seems suitably cruel - demonic, even.

As befits their nature, of course.