My orrigional thought was that in the world of D&D injustice is definable because the game mechanics need it to be to work. They rely on black and white ideas of good and evil. Some monsters are by definition evil, and some creatures are by definition Good. Smite evil works because everyone knows what it is going to smite without having to worrie about perspectives getting in the way. An evil villain might think he is just in his actions, and he has a very good reason to burn down a villag full of innocent bystanders to get to his intended target but even with his warped sense of moral compass that vaillain is still evil.
However having thought on this, injustice requires evil to work (even a good character carrying out injustce is doing something evil) and the dark powers wouldnt let you scry for evil, so meh, egnore that one.