Presence of an evil inclination as well as an inclination to good inthe human person (Jewish rabbinic psychology; Freud's Thanatos/Eros hasa certain resemblance to this)
Original Sin (see, e.g., Romans Ch. 7)
Inertia/laziness (see M. Scott Peck's The Road Less Travelled.
Various social psychological dynamics gone awry (for material suggestiveof this, see Robert Cialdini's Influence).
Ernest Becker's theory of death denial and heroic nisus (see, e.g.,Denial of Death and Escape from Evil.
Vonnegut's Mother Night suggest that the tendency to demonizeothers is the source of great evil.