“I’m not suspecting anyone. I’m only trying to understand what can have happened. In any event, you tell me this took place some years ago, and it’s odd that anyone would steal a poison and then not use it until so much later. It would suggest a malignant mind brooding for a long time in darkness over a murderous plan.”
IF MY SUSPECT AIN’T GUILTY I SWEAR!!! (Edit: damn, I finished the book. That line "brooding for a long time in darkness" is a little on the nose now that I recall the ending)