Explanation :
Arsenic and old lace is used in rat poison and is found in the title of the murder mystery.
Arsenic is a poisonous element. It is a metalloid that has some properties of metals and some of the nonmetals.
As it is a toxic thing. It can attack the digestive system, nervous system and on the rat's heart. If it is consumed by rats or humans in excess amount, the rat can kill very quickly.
So, Arsenic and old lace is the element.