Why, it's the opposite, Master! There's no trouble here whatsoever. I should only hope my reticence in sharing my True Name comes as no difficulty to you.

[ He is absolutely just letting her work it through on her own. He's already at the point of discovering whodunnit, now he's debating how fun it would be to let them all escape for future shenanigans versus how much capitol he'd lose with Ritsuka if she pieces it together. ]

All of the staff, likely starting with any who would have means to murder in this way. That is to say, with either a knife, a blunt object, or poison.