We might think of words such as ‘courage’ as naming characteristics of human beings in respect of a certain power, as words such as ‘poison’ and ‘solvent’ and ‘corrosive’ so name the properties of physical things. The power to which virtue-words are so related is the power of producing good action, and good desires. But just as poisons, solvents and corrosives do not always operate characteristically, so it could be with virtues. If P (say arsenic) is a poison it does not follow that P acts as a poison wherever it is found. It is quite natural to say on occasion ‘P does not act as a poison here’ though P is a poison and it is P that is acting here. Similarly courage is not operating as a virtue when the murderer turns his courage, which is a virtue, to bad ends. Not surprisingly the resistance that some of us registered was not to the expression ‘the courage of the murderer’ or to the assertion that what he did ‘took courage’ but rather to the description of that action as an act of courage or a courageous act. It is not that the action could not be so described, but that the fact that courage does not here have its characteristic operation is a reason for finding the description strange.
-- Philippa Foot, 'Virtues and Vices', p.16.