Evil doesn't exist. It's a value judgement. We have an understanding of certain specific instances of certain specific acts being "evil" (which ones and to what degree depending on various sensibilities but for acts involving physical harm at least most humans share a common ground). We also have culturally variable codes of law (religious or secular) which likewise define categories of acts as "evil" or illegal and often defer to scholars (judges and priests) for edge cases.
But unless you literally believe in evil spirits, demons or devils, "evil" is not a thing that exists, it's an emergent property of human behavior and as such can be explained by pyschology and sociology (e.g. as in the Milgram experiment where "evil" is the product of the participant's desire to comply with the experimenter's instructions to the detriment of a fake subject separated from both by a wall with an intercom).
Studies of human behaviors in crisis situations such as after natural disasters causing a collapse of infrastructure and social hierarchies likewise demonstrate a tendency towards mutual support and solidarity, quite to the contrary of what popular media would have you believe as the norm in the "post-apocalypse".
It seems that human nature (because humans are a social species, i.e. we rely on each other as a group rather than natural defenses of our own bodies) is cooperative and it takes social hierarchies and complex systems in order for us to do "evil". Heck, a huge part of military training ever since the modern age consists of dehumanizing the enemy to overcome our natural hesistation against causing harm to other humans. Even the nazi police officers who volunteered to participate in the mass killings of Jews in Poland mostly came up with narratives for themselves to justify their actions as humanitarian (e.g. we know of one such officer who insisted on only shooting children and waiting until their mothers were killed by his comrades so killing the child was an act of "mercy" - turning what was clearly an "evil" act by anyone's standards into something he could justify to himself).
