An interesting read,
Wikipedia entry on "evil"
Some philosophers believe that only acts can be evil, people cannot. In other words, a person may
commit evil acts, perhaps many of them, but cannot
inherently be good or evil. I know that swings against the D&D alignment system, but I thought, in the light of this discussion, it might be something worth considering.
The closest thing to real evil in our world is the concept of the sociopath. These people have no psychological connection to the concept of good or evil, and have no emotional reaction to committing so-called evil acts. Of course, such people do not feel that what they do is "evil", but necessary, for whatever reasons. Many of the most "evil" people (or more correctly, the most egregious of sociopaths) feel compelled to perform "evil" acts (whether mentally compelled, physically compelled, or feeling a social duty to do so).
If you must include spells like Detect Evil in your games, and you don't want to use alignment, you can still detect evil intent (in the midst of performing an evil act, or perhaps intending to perform an evil act). This makes much more sense, and prevents your Paladins and such from trying to smite everything that momentarily pops up as "evil". After all, just about everyone performs an "evil" act now and then, if only an act of selfishness that goes against the social mores.