The whole concept of "I kill you because gods let me see your red aura" is evil.

"I am wary of you because gods let me see your red aura" would be fine RP-wise.

OR it would be fine if it was stated that you can't get Red Aura just because you're "inherently evil", you get it for committing atrocities. Like if you choose Evil on character creation, it means that you, sir, have committed acts of rape, murder, betrayal and maraudering (altogether) and have made them part of your character through those 16 years when your character's lifetime starts. That would be fine as well.