Evil-as-absence works well from a virtue ethics philosophy. Absence of love, compassion, honour, generosity; the more virtues lacked, the more evil.
Evil-as-harmful works well from a utilitarian philosophy. That which does the greatest harm to the greatest number; and the easiest way to slip into that is to either have a character who only cares about what benefits themselves, or to have a character who only cares about outdoing others ("It is not enough to succeed; it is also necessary for others to fail"), or to have a character devoted to a cause which happens to be very harmful to those around them (eg an Asmodeus cultist, or someone from the former ruling classes who wants to regain their former power at any price)
Deontological evil (evil as rule-breaking) doesn't work quite so well IMHO, and also slips into the Law vs Chaos split in this game world... and although a chaotic evil character devoted to breaking all order and structure would be interesting, there might be issues with simultaneously being an agent of Asmodeus. But an absence of being bound by the normal rules of decent behaviour might work.
Then, of course, there's Evil as the fundamental force of the universe that it is within this world's cosmology. But that's more of a structural thing (eg a diabolical tutor) than it is an intrinsic motivation.