The problem is that there's no incentive to be evil in most games aside from the novelty of being evil, which is a pretty meaningless and childish way to view morality. Give people a hard game where a time-limited quest in the first chapter has you delivering a +5 sword to a paladin defending a village and people will start considering the evil side worth taking. But no, games have to be easy where all decisions are equally valid and "balanced".
Not to mention that evil is usually portrayed without nuance (i.e.
stupid evil). KotOR being one of the bigger culprits in this regard.
Evil should be portrayed as ruthless people doing self-serving egoistical things, or as ruthless people who genuinely believe their cause is right and will do anything to achieve it, no matter how extreme and cruel.
Usually it's portrayed as just being a spiteful dick for no reason. "Ha, ha, ha! I kicked this puppy because it looked at me with cute eyes, and I despise cute things! Ah, to spread misery is such fun!"
Here is how evil is portrayed and rewarded in your typical IE or whatever game:
Thank you for saving the fair princess! I must reward ye:
- No, I don't require any reward. (+good, +lawful, +3 ring, +1000 party xps.)
- Yes, I traveled far to save yon princess (+3 ring, +500 party xps.)
- Surely you understand I went through considerable risk (+chaotic, +3 ring, +250 party xps, 300 gold.)
- Yes, a reward had better be forthcoming, or next time I'll leave her to rot! (+evil, +250 party xps, 300 gold.)
Maybe a better way would be a more manipulative approach using dialog. That is, someone who is evil doesn't go around saying "I'm evil, look out." Evil people I know say or do whatever it takes to get what they want, and don't spend a lot of time thinking about the moral ramifications of their actions, nor spend a lot of time thinking about future implications of actions. I have a friend who sells used cards. He would be neutral evil in the D&D sense, but I doubt he's ever stopped to think about it. He just is. Being evil in games just doesn't pay off, but maybe it should to balance out the long term negative ramifications.
Ironically, the highest rewards in most RPGs are for dialog options that say "No, I don't require a reward." Maybe a better way would be:
Thank you for saving the fair princess! I must reward ye:
- Truth: Nay, I don't require any reward. (++good, +lawful, +1500 party xps.)
- Yes, I traveled far to save yon princess (+3 ring, +500 party xps.)
- Surely you understand I went through considerable risk (+chaotic, +2 ring, +250 party xps, 300 gold.)
- Lie: Though I have taken a vow of poverty, anything you can contribute would help me protect those less fortunate (++evil, +3 ring, 1000 gold, +1000 party xps.)
I'm saying that most folks savescum for the best reward - well, what if the best option was manipulative and evil, instead of the best option being good? What if being good and not wanting a reward led to folks saying "Ok, chump."
The truth and lie thing is something I really liked in Planescape Torment. Practically it made no real difference, the NPC would believe you either way, and in the end even if you picked [Lie] you could make true on your promise. But doing that would raise your alignment towards Chaotic.
PST generally works with the D&D alignments pretty well. I just checked a wiki to see what actions and dialogue choices turn you towards evil in PST, here's a selection:
- vowing not to reveal someone's identity to the authorities, but then going ahead and doing it anyway
- lying to Deionarra because it's manipulative
- there's a guy who wants to die and asks you to kill him; demanding money for it adds evil points, going ahead and actually killing him (no matter if you asked for payment beforehand or not) further awards evil points
- threatening people with death unless they do what you want them to do
- general lying to people (even if it's something as basic as saying "I don't know where the bar is" when you've already been there before... I think that should raise chaotic, but not evil)
- asking people for money in exchange for performing life-saving quests for them (again, not a fan of that... even policemen and professional firefighters get paid for their job, asking people to pay you when you help them isn't evil)
- tossing Morte back into the pillar of skulls
- selling your own companions into slavery
While there's still some dumb stuff (why do white lies contribute to evil? why does demanding payment contribute to evil?), many of the evil options in PST are actually evil. And some of them make sense for a power-hungry character to do, like doing the bidding of that one grimoire that grants you magic power if you do its bidding and sell one of your companions into slavery, and murder another.
EDIT:
For your specific example, I think the best way is to give the same amount of XP for each dialogue choice. The experience is granted for saving the princess, not for your attitude about it. Even if you're an ass, you saved the princess. That's the important part. That's what grants you experience. RPG conventions like experience are supposed to be abstractions of real life stuff, like getting better at doing things by successfully completing adventures. If you complete a lot of adventures, you get better at adventuring - logical, isn't it? That's how it works in real life too.
If you fashion a hundred tables you are gonna raise your carpentry skill that way. It doesn't matter how much you sell them for. Even if you rip off your customers you're gonna get carpentry XP. Only the tablemaking itself matters.
You could rape the princess after saving her and then kill her, and it would still be reasonable to get full XP for it.