Banality of good

The banality of good is a concept of Christian ethics where a little evil outweighs much good. Ecclesiasties 9:18-10:1 Wisdom is better than weapons of war, but one sinner destroys much good. Dead flies make the perfumer's ointment give off a stench; so a little folly outweighs wisdom and honor.

"The toil of all that be

Helps not the primal fault

It rains into the sea

And still the sea is salt" - A. E. Housman

Analogies

Poison in the cup

Biocides are powerful, but the effect that they have is only negative.  

Game theory

The prisoners dilemma and Nash equilibria.