Are you sure you really can't? It is often enough to just look at the person to see. And that are not only eyes that say.
But, sure, I agree, stating that:
- ohh I can say is he good or bad 100%, it's enough to just look at him -
is very childish and wrong. Nothing is 100% in RL.
This is much more complicated matter, as people use "masks" to hide their true nature.
But this is game. In a game/fantasy world attitude or nature can be underlined. It can become more visual, depencible on actions. Purpose? - fun, replayability, interaction.
And auras will not hurt at all
It is a concept. Thief will not be able to walk on the streets of the city during daylight. Citizens may recognize him and become hostile/call for guards.
He will have to sneak at night staying away from big streets. He is silent asassin, he doesn't has friends, his only friend is poisoned dagger, his best ally is night and shadows (and good examples are - Splinter Cell, Thief series).
And as far as reputation is concerned, you are absolutely right. Thief may have high reputation among thieves.
Can thief have honour and be therefore respected among people of honour?