The concept of good and evil does not formally exist beyond humans, and even within humanity there is no universal good or evil that can be agreed on." Objective morality" is the evolutionary byproduct that allowed humans to stick together since the beginning of their existence without rampantly killing each other for stupid reasons becuase back then, the lack of prosocial behavior (AKA sticking with and cooperating with other humans) would mean that you were dead meat. Of course, this trait is not perfect nor is it universally present in every human. That is why we get murderers, rapists, etc. etc. But when it comes to what is "evil" and what is "good", that is mainly hinged on what the individual's disposition and what they believe in. Humans are far more intelligent than any other animal on this planet by a wide margin. We need a moral/ideological center, regardless of what it teaches or where it is from, to live around and roughly abide by. Of course, one of the more universal maxims of humanity is that saying that hurting or killing others is bad thanks to the existence of said "objective morality". Religions like Christianity are a good centerpiece for many people because at face value it is simple: do not kill, do not hurt others, do not steal, etc. etc. However, these centerpieces can easily fall apart/can be exploited by the very people that orbit around it, and sometimes these centers themselves can be completely divergent from the "objective morality" most humans inherently have. These blips or absolute differences are what many people on the recieving end call "evil". The muslims were considered evil to the crusaders, while the muslims considered themselves as good. Hence there was conflict. The Nazis viewed hitler as good while other groups such as the jews may have not viewed hitler in the same light. Humanity will never agree on what is universally good or evil likewise unless they are a hiveminded society. Even groups like females who are seemingly bland and uniform have differences that lead to this type of similar behavior.
The centers themselves are also much more ambigious than people might think. The truth is anyone can create a center and not everyone has to follow a center that was already created by somebody else. That is the reason why we have different christian denominations, islamic branches, and other religions/ideologies that often view each other as evil. These centers can also be self-radicalized and then orbited around as simple as that, and can sometimes appear to not have an organized structure at all. Regardless, that is why even when the majority of people see extremists as "objectively evil", The perpetrators themselves and sometimes people that believe in similar ideologies usually have motives and/or beliefs that made them believe their actions were good or otherwise justified to not be "evil". That is why when you identifiy something as "evil" or "good", it's likely from the bias or yourself and/or many other people instead of the collective belief of all of humanity. You can view followers of another centerpiece to be evil, dumb, stupid, smoothbrained, etc. etc., but there is no denying the fact that what you call good and evil is not necessarily the same good and evil they will believe in despite there being the possibility of similarities. Even when you look at literature since the ancient times you can easily see that the brand of "evil" is often skewed in favor of the protagonist. In The Odyssey, the Cyclopes violates Xenia, which is the ancient greek law of hospitality, but to be fair Odysseus intruded on the home of the Cyclopes, stole his livestock, and refused to leave. In the perspective of the Cyclopes, Odysseus is as evil as the Cyclopes is evil to Odysseus. Both perspectives are biased and both perspectives have belief that they are in the right. Even in modern media, as horrible as some of them (EX: Hazbin Hotel) is, ironically shows how differently you can view things that were traditionally branded as good and evil by society, because all of them at the end of the day are concepts made by humans with different centers to their personal beliefs.
