I like to think of good and evil as seeds planted in all men's hearts, as pods inside us that will grow roots and spread if nurtured.
I like to think of the societal rules under which we live as a type of fertilizer for these hidden seeds. In societies in which facts are valued more than opinions good is nurtured and evil withers (meteorology replaces virgin sacrifices). In societies in which Justice is valued more than punishment goodness grows and evil weakens (courts replace witch-hunts).
Conversely, when you have a US president continually promoting torture it is a type of cultural fertilizer which makes evil weeds from evil seeds mature into evil deeds. Justice is then perverted into punishment, Beauty into a prostitute, tolerance into bigotry, knowledge into opinion, intelligence into ideology, and community into angry mobs.
And... soon enough evil weeds begins to spread in our communities, in our bedrooms, and in our schools.
As the weeds gain strength they begin to choke off the plants which bear fruit. Voices of reason fall on deaf ears (global warming gets ignored), Voices of compassion fall on deaf ears (Darfur gets ignored), Voices of humanity fall on deaf ears (Guantanamo gets ignored), Voices of Justice fall on deaf ears (Habeas Corpus gets tossed), and the eyes of men grow dull to Beauty (witness suburban sprawl).
Some version of this mental exercise has been put forth over the ages through Philosophy, through Religion, and through Art. The message is this:
Honor the light or live in darkness
Worship the good or be ruled by evil
Watch the garden or lose the fruit
In the Bush era the bees leave us.