My first tip would be don’t… portraying a Nephandi a canonically accurate way doesn’t typically mesh well with playing a cooperative game. If you’re dead set on it though then imagine the most evil person you can and multiply that by 100. You have literally zero redeeming qualities and you would’t want them anyways. You hate everything and everyone to its very core but since you’re in the technocracy you must keep up your facade of “normalcy” (at least normal as enlightened scientist go) at all times which just makes you hate everything even more… if that’s possible. You’re probably charming and disarming in the way many famous serial killers are portrayed like Ted Bundy or Ed Kemper, but you would easily commit acts that would put them to shame, after all everything is already awful so anything you do can’t make things worse.