Hemlock
Oenanthe crocata  and several other species of the water dropwort are extremely poisonous. It has been referred to as the most poisonous of all British plants, and is considered particularly dangerous because of its similarity to several edible plants. The active poison is oenanthotoxin. The poison produces the "sardonic grin" a spasm of the facial muscles that produces what appears to be a grin. The poison was said to be used in ancient  Phoenician Sardinia to kill old people who could no longer support themselves.