This contains spoilers for Sunday Without God.
Ulla Euleus Hecmatika is the princess of Ortus, the capital of death.
She is the Idol of Murder, and whoever she sees, talks to, or touches, dies. Supposedly, she doesn't know this, as she's been fooled her whole life.
Kiriko, her instructor, was asked by some elders/superiors/whatever to fool Ulla.
What were they expecting to accomplish by lying to her?
Not from Kiriko's point of view, but rather from the elders/superiors who asked him to do it. Why do they want to fool Ulla? What were they attempting to avoid had she known the truth?
I suppose that she could get rebellious if she knows that she's murdering newcomers who want to join Ortus, but the concept of death has blended too well into the world's culture for over 15 years, so it is a daily, trivial thing and I can't imagine the elders considering it a bad idea to explain it to Ulla.