I'm no philosoraptor nor have I played one on TV, so I'll try to do my best here.
"Good" and "Evil" are, in a certain sense, directions on a scale, or on many scales, which themselves do not need to be any well-defined. The real utility of good and evil as concepts is as shorthands to describe the knowledge or sensation that doing this or that action will tip a scale of relevance in a direction that makes a situation better or worse (with those terms already ill-defined) for an afflicted person or entity. As such, we define welfare for example as those measures that will improve a person's basic living situation, and we could as well define "worsefare" as those measures that do the opposite.
For example, it may be debatable that killing people is a bad thing. It could be because there is no agreement on the scale that is being used. On the scale of "how do I properly defend myself against an attacker using lethal force", for example, and barring any other more constraining considerations, killing is a sort of "ultimate good turn" - the ultimate assurance that the opponent will stop their lethal attack and turns the would-have-been-victim's situation in a safer direction than it was before. However, this does not make killing in this scenario itself "good" - it merely makes it (once again abusing the terms) better than the alternatives.
Now, this sort of evaluation works really well in a schema of moral or ethical evaluation that is defined to be of one-person conflicts, such as the ones used in some stereotypical ethical tests like the Prisoner's Dilemma, because they do lack the "more constraining considerations" element. It's just a man versus a situation in a sort of moral and ethical void.
But as we start aggregating more people, first one, then two, then the thousands in a town, then the millions in a city, agreeing to which scales to use and to which constraints to submit becomes harder and harder until a point is reached where it is not possible, anylonger, to define a scenario where a response that is considerd good or evil is unique, or at least well-defined enough to be useful- supporting welfare for migrating populations is a very good early culture example. The only salvation of sorts tthat provides a regular framework is that enough people agree on which scales to use and then codify those as a pre-requirement to engage in social construction. The results are what (best case scenario) we usually call "Ethics" and "Law" depending on who does the codifying and how it is established.
With all this, it's very difficult to define what a True or Absolute Evil would be - ditto with a True or Absolute Good. The only regular definition that I can approach to is that a True, Absolute Evil is an act or a trend that turns as many socially desirable scales to a worse condition as possible, or that tosses enough scales into a worse enough condition that it can not reasonably be reversed. For example, a so-called "President" that ignores any and all senseful advice possible and continuously and strenuously exposes their population to large to a medical threat that results in the un-improvable situation of them being dead so that a selected few can further perfect the exploitation of those who would remain alive in worse condition, skirts very close to an absolute evil in this scenario because it tips the scales of living to the worse unambiguously for everyone except a select few who, to make it worse, didn't need the improvement anyway. (I'm not calling out any Presidents or billionaires in particular because there's so many doing this. If the shoe fits, it's on you)
However, for that definition to work, the many scales have to be clustered or partitioned according to an external objective, prescriptive criteria that the tippings themselves can not affect: for example, geographical location, time of the year/century, age of the afflicted. And because those variables also define culture and ethos, it turns out that out of mere statistical necessity of there being too many people to count their options, the higher concepts of what evil and good can be are defined in terms of their particular culture at that particular time and can not in any way be considered absolute.
Ditto of course for a True or Absolute Good, only reversed in the direction of the scale-tipping.