Presumably that's the first thing dark lords (and their real-life equivalents) convince themselves of, that there is no inherent good and evil.

The hell is the real-life equivalent of a dark lord? Can that even be addressed without getting into discouraged topics?

Also, "convince" implies not only intent but that the individual started with a different belief, maybe even that it is universal to start with a belief in good as evil. That sounds like a couple of unwarranted assumptions.

On a personal note, I once expressed the belief that there was no good or evil. I did so privately because I well understood there are undesirable consequences of sharing that belief. Before that time I had spent much thought over much of my young life trying to make sense of the concepts, to define them in ways that were consistent and useful, and was constantly frustrated.

I did not convince myself that there is no inherent good and evil so much as I gave up on trying to convince myself to believe otherwise. I expect a fictional 'dark lord' or real-life 'successful and wildly powerful individual of objectionable character' could as easily experience the same surrender among a larger number of alternative ways to leave good and evil behind.

(On a further and more indulgently personal note, I've since become disinterested in any requirement for good or evil to be 'inherent:' good and evil do not need to be applied in a perfectly consistent fashion in order to be useful. And it happens that I am evil and likewise disinterested in being good for goodness' sake.)

Once that part is over with, anything you do can be classified as good.

I may misunderstand this due to one or more philosophical shortcomings, but if why bother classifying anything as 'good' if you've left 'good' behind?