"We killed the people saying our "Holy Water" doesn't work perfectly as a cure for the lung disease because the know people who died even after drinking it. Now we're looking for the source of rumors saying our water doesn't work, and our source said that a case of advanced lung disease was cured pointing to you as the source of that cure. You have to teach us the way to manufacture the cure you made."

If I were the MC I would've immediately dropped all pretenses and just told them NO. I would even tell it straight to their faces that they are evil and not worthy of the knowledge. I mean, they're so convinced by their "holy water's" efficacy that they would kill people just to keep the detractors quiet. If they believe in that thing so much, they should all die with it. Who knows what they'll end up doing with the knowledge to manufacture all sorts of drugs, and how many more people they'll kill just to keep that knowledge secret.