I feel like D&D has always focused on simulating the
PCs with everything else being various levels of cheats.
Think of computer games where the AI 'Cheats' because it has perfect knowledge while the player deals with fog of war and limited knowledge. That tends to be D&D... And it works as long as you don't look for the crack too much. A lot of stuff (Raise Dead) was put in to fill a need for dungeon-crawling characters with no real plan for exploring a world with that kind of ability in it. Some editions have dabbled with making monsters work more like
PCs (3rd, for example) which had good and bad effects. 4th pulled back from this hard with a design concept that monsters exist only to interact with
PCs... Anything that happens when they're "off camera" is there for the
DM to fudge as needed. So monsters don't get dailies (they're only in one scene of combat, generally) and work ona different 'economy' than the players. It works in practice, even if it sounds ugly.
A favorite hack of mine for D&D is changing Detect spells to make 'investigation' more interesting. The core of the idea was borrowed from a White Wolf World of Darkness book, but this is adapted for D&D. Basically, Detect Evil only shows evil on innately evil things (Demons and Devils) but does leave a lingering 'sense' of evil in an area where evil acts were committed, even on a victim of an evil act.
So if someone is beat up by an evil character, they'll show as 'evil' for a while. The sense of evil would linger longer the more unpleasant the evil is: severe emotional abuse is going to leave scars that'll take a while to heal. Obviously, players would need to understand this change.

Paladins wouldn't be able to pick the evil guy out of a lineup so easily, but would still know they're needed here as something evil is happening. They might be able to travel around a keep and find the room where the evil guy hangs out, especially if he does bad things there, but it's not as cut & dried as standard
Detect Evil.