Notions of good and evil run through the heart of the Judeo-Christian mindset. Admittedly not so much in Eastern religions, but western civilisation derives from Judeo-Christian thinking and these concepts are still alive and well. You will often see our own Mr Burns for instance describing the world in terms of a battle between the forces of good and evil being played out through human lives. I find this simplistic nonsense worth challenging with a more realistic rationale wherein all behaviours have roots and understanding those roots will eventually yield better outcomes, rather than simply labelling people shortsightedly as 'evil'.
The extent to which religious concepts affect and channel your thoughts (albeit negatively) is amazing....
Evil (immoral, wicked) exists. We can't shy away from that merely because Christianity talks of evil!
As I have said, much of what we consider as Evil could be related to animal instinctive behavior that is unacceptable in human societies. But there is much (listed above) that lies outside animal instincts. There is more evil in humans....in the sense of enjoying the suffering of others....than in animals.
It is simplistic to attribute all this to mere pathological reasons. Evil is not just a disease.
There are probably genetic reasons for evil. Circumstances and upbringing alone cannot be responsible for evil because there are many people who live in very poor and oppressive circumstances but are very good ....even saintly....people. It is peoples nature of reactions to their circumstances rather than the circumstances themselves that result in evil.