It is pretty sad that the religious and the non religious seen to need to contest with each other which of them can create the worst human beings when history is replete with hideous examples of both. One would think in that case that an alternate explanation of human evil would be sought as the cause. I have offered an explanation as to why this is so over and over, but will do so again.
The source of human violence, one against another, lies is the unexamined assumptions we make, that are grafted violently onto the genetic nature our evolution has given us, a profound capacity to love each other and to cooperate as a group and our territorial fear of the other. Very young and thus preconditioned children display both a high percentage of empathy for others but a fear of different appearing others. Culture and education the nature of ones exposure while growing up can influence positively or negatively these separate tendencies. A bigoted insular culture that is exposed to constant warfare or other threat will develop a culture of fear, and a culture of largess will be much more loving and welcoming.
In a stressed culture group cohesion becomes a necessity of survival. Great demands are put on children to conform to a group think, to adhere to distinctions that demark one type of group from another. The methodology by which this cohesion is maintained is hate of others, and the threat that you will be hated too if you do not conform. Thus is born group pride and group self justification, the right of the self to affirm itself over the selves of others. This is the evil of ego, the certainty that only oneself and ones group are right and that right and might are one and the same. This is also the origin of self hate, the naming of what is right and its forceful application onto all the newborn in that society who are born whole and perfect and not yet brainwashed into that culture. That original self we are taught to despise so we can conform and take our place as worthy members.
It makes not the slightest difference of one belongs to a theistic or non-theistic culture. Once the certainty of ego is fixed anything can be justified. It is this ego of certainty that is the ultimate evil, it's ability to justify anything.
This is why a modest Christian is vastly superior to an egotistical Atheist and a modest Atheist vastly superior to an egotistical Christian. The origin of all evil is self hate and ego is its face.