Evil is unintentional, not an essence, and has an accidental cause which is good.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“Saint Thomas’s position is that of Aristotle. Universals do not subsist outside the soul, but the intellect, in understanding universals, understands things that are outside the soul. The Third Book is largely concerned with ethical questions. Evil is unintentional, not an essence, and has an accidental cause which is good. All things tend to be like God, who is the End of all things. Human happiness does not consist in carnal pleasures, honour, glory, wealth, worldly power, or goods of the body, and is not seated in the senses.” source