I would like to see a spell that works like Detect Life, only makes characters glow with a blue greenish aura if they won't attack you on sight, and a reddish aura if they are hostile and will attack on sight.

??? Detect life does do that doesn't it?

Detect Life already does this. Enemies Glow Red, non hostiles will glow blue.

Haha nice. That'll teach me to request something before I test out what I already have! =P

Though what about an npc who won't attack you at first, because like all good villains they must reveal their dastardly plans to you in detail before they proceed to attack you.

Will such an npc appear as blue or red?

An example would be a certain nighthag for a certain quest, at a certain location.