That's a narrower definition than I've usually seen.
I consider it a far broader definition than you have probably seen.
D&D writers have had a notoriously hard time defining morality.
…isn't "evil" usually defined as greedy and selfish....
Sure, but why is "greed" or "selfishness" actually evil? What is wrong with it? I put forward that the problem with them is that they are destructive, and to the extent that they are not destructive we wouldn't consider them evil. For example, while greed is frowned upon, a person who is thrifty and prepares up a store to survive a hardship is admired, and while "selfishness" is considered wrong, a person whose self-interest benefits themselves and does not harm others is likewise not considered evil. It's only if you take at another's expense, that we see it as evil. Mutually beneficial exchanges of labor or other partnerships, while they might be motivated by self-interest, aren't seen as evil but good, since there is a net profit - that is, a net growth and creation. People can and arguably should marry for self-interested reasons, but as long as there is a mutual profit and benefit, the marriage is healthy. In short, things like "greed" are only evil if they are destructive. Specifically, greed is the evil of hoarding something not for use, but for the pleasure of withholding it from someone else. Consider for example the example of Ebenezzer Scrooge, who though he is immensely wealthy, never spends any of his money for his own pleasure. He's merely satisfied to think to himself how much more he has than others, even though he himself lives in near poverty. It's this holding something while withholding something from a good use that is "greed", and hence it is destructive. His merely being wealthy was not greed - the 19th century reader would for example immediately recognized that he contravened the normal social order by being both wealthy and yet employing no one. His wealth was a dead end, that didn't return to the community. Also note that the origin of his wealth is not any sort of productive activity, but simply rent seeking and arbitrage. He doesn't make anything. He just takes.
Finally, note that we also consider many acts which are without self-interest to be evil, if the harm is done to the self. The shared problem is not self-interest, but destructiveness, whether self-destruction or harm to others.
Even genocidal warlords, however, typically want to replace one ethnic group with their preferred ethnic group, so they're not just destroying everything because they want nothingness.
Sure, but the desire to replace a diversity of tribes with your own tribe, and the belief that the entire meaning of existence is simply power, and the general notion of nationalism without limit, is strongly associated with the idea of "Lawful Evil". That is to say, it's not Pure Evil, but mixed with the notion of "the needs of the many" in a way that is a blend of both the goals of evil, and the goals of law. Thus, lawful evil generally does not see "the needs of the many" as being associated with weal, since weal in the minds of Lawful Evil types brings softness, love of pleasure, and other sorts of social illusions that allow people to forget what life is "really about".
