I think he just assumed that the number of people who are actually capable of cold blooded murder in public are very few, and that the number of people who know

I think he just assumed that the number of people who are actually capable of cold blooded murder in public are very few, and that the number of people who know how to get and use poisons properly is even smaller. Everybody can come up with a movie plot scenario where something is possible, but the actual risk of him dying was actually very small.