In the extreme cases there are individuals who deliberately and coldbloodedly commit evil in full knowledge that what they're doing is wrong. They tend to be the ones manipulating things within cults and other social groups promoting pathological conditions. In many cases they are genuinely self-deluded but its not all together uncommon for them to be perfectly aware of their gleeful malice. Whenever given the power or opportunity they will cause misery and havoc on others.
Then there are the 'sheep'. The 'sheep' are not essentially malevolent themselves, but they tend to fall victim to the manipulation of socio-pathogens and end up allowing themselves to become crude extensions of their will [e.g. the Manson 'Family']. When confronted with the evil of what they have done/are doing they will often rationalize it away, lie to themselves, resort to denial or any number of other psychological defense mechanisms to avoid taking responsibility ["I was just following orders!", "I didn't know -- Really!", "That never happened!"]. The "sheep" aren't essentially evil themselves but, thru their psychological weakness and cowardice, they become accessories to it.
Events like the Holocaust and the Killing Fields happened because there was a critical mass of psychologically weak, desperate people. Also present were unscrupulous bastards who, like opportunistic infections, were there to take advantage of that weakness to con them into making their societies even more hellish. Though there are different degrees and varieties of evil, there is evil none the less.
