Yes, a spell with the [evil] descriptor is an evil act.
Neutral casters have more flexibility, but no paladin could ever cast it, and I would determine that a good cleric would also not be given access to that spell by their good deity.
Evil for detect and smiting purposes has to do with character alignment, not spells known/cast. A neutral caster could know and cast evil spells without being evil, and therefore would not be detected as evil and could not be smited (smitten? that just sounds wrong...).