Even the whole "then vs us" thingy is not inherently evil, since both sides see the other as bad and themselves as good.
Not really. Many evil people know they're evil.
It's just they put their own goals above the act of "Being good"
There are some that do believe they are good, but there are a lot that know they're not.
How do you even role-play an utter lack of empathy when you still have some empathy running around in that brain circuitry?
Being evil doesn't require you to be devoid of empathy. You just have to care more about personal gain than the feelings of empathy.
As far as role-playing it goes, an easy (And fairly common) way to do it is by reducing ones own empathy. By way of acknowledging that the NPC's in the game are just that, NPC's. A collection of pixels with pre-recorded dialogues implemented into a virtual game.
Ergo, realising that you don't need to empathise with pixels and "Harming" them is meaningless because they don't exist and have no ability to feel anything.
Quite literally the entire point of good writing in media is to allow people the actual ability to empathise with characters. People are in a natural state of not empathising with fictional simulacra. It is the job of writing (And actors) to try and convince people to actually consider characters to be real enough to empathise with.
Hence things like how I can mow down civilians in GTA without batting an eye. While it feels like a knife in my chest whenever I have to kill Angel in Borderlands 2.
The former have no writing or acting to make me consider them anything but soulless husks. The latter has half a game worth of interaction and great voice acting to develop a sense of life.
Interestingly this concept is also a thing that some people do in real life in order to commit evil acts, they dehumanise people so that their empathy doesn't kick in and so they are capable of committing atrocities (It's also surprising how quickly and effectively this can occur. The Stanford Prison Experiment in 1971 showed that very clearly)
