Giving exponentially more XP for higher level enemies and requiring exponentially more XP for each additional level would accomplish the same thing.
-_- Really people? Really? This is already a thing: just look at game one
And here's the complaints: "Evil" = more exp = one/two extra levels for hours more work and multiple playthroughs and guides
You are right. I guess the system doesn't really need a change.
Neither peaceful nor killing are always better than the other option. Picking the optimal reward every time still doesn't result in a significant level difference, after any amount of time. Developers should probably leave as-is and focus on more important things.