I find it extremely difficult to frame scenes where interesting choices can occur, under the rubric of (as noted) genuinely unrepentant evil. Most things that fall short of that, I have something to work with. But someone who is consciously wicked, and has no desire to change that (and where player intent is that that not change)...I just start drawing blanks. I don't know how to speak to that. How to give it interesting scenarios--other than ones that would, in one way or another, be pushing away from genuine, unrepentant evil.
While it may be more than the color of an outfit, surely it is much less than a (by player design) baked-in desire to do wicked things, particularly one that is (again, by player design) not amenable to change.
If I had to choose one of those two to declare which one "reptilian physiology" was closer to, I would absolutely choose "color of outfit," rather than "fundamental values and behavior."
I guess my first question is,
How do we know this character is incapable of change or repentance? Wouldn't that be something to be revealed in play?
More generally, I'm somewhat puzzled why it would be hard to come up with interesting scenarios for a character for whom (to quote Gygax)
purpose is the determinant (ie they do not recognise moral obligations to others, nor ethical demands on their own conduct). Any sort of traditional logistical or means-end puzzle would probably do. For something more character based, Citizen Kane or Treasure of the Sierra Madre could provide ideas.
If, rather than Gygaxian tradition, you're going with 5e alignment descriptions, here are some possible starting points:
Lawful evil (LE) creatures methodically take what they want, within the limits of a code of tradition, loyalty, or order. Devils, blue dragons, and hobgoblins are lawful evil. Any sort of thief-oriented scenario would seem to work here; BitD probably has a lot to offer.
Neutral evil (NE) is the alignment of those who do whatever they can get away with, without compassion or qualms. Many drow, some cloud giants, and goblins are neutral evil. Seems well-suited to dungeon-crawling, looting, politicking, frankly any sort of self-aggrandising activity, which D&D features quite a bit.
Chaotic evil (CE) creatures act with arbitrary violence, spurred by their greed, hatred, or bloodlust. Demons, red dragons, and orcs are chaotic evil. This sort of character seems more like they would be a hanger-on or "secondary" character, rather than the principal protagonist. It seems like a barbarian, monk or fighter could all work well as CE (a MU or cleric not so well, insofar as they are more likely to play a dominant role in guiding the group's actions).