I mentioned it in the other thread, but I think it bears repeating for this thread.

The trouble is that what constitutes "Good" and "Evil" is subjective. Even if the concepts are concrete and objective within the game*, the game itself is played in the framework of our subjective reality - hence, the concepts themselves become subjective.

In other words, what defines "Evil" changes, and while there is some common ground, there is no way to be sure that everything one person believes to be "Evil" will be considered "Evil" by a second person - and when one of those people is the DM and the other is a player, the trouble starts right then.

* - Note that I said "the game", not just "D&D". Even if the game doesn't use an alignment system, the concept of "Good" and "Evil" will exist in most any RPG you can mention.

Particular to D&D, the alignment system has people thinking "I am X alignment, so I must do Y behavior," rather than "But I do Y behavior, I am X alignment." The alignment system is supposed to be a guide to allow the behavior of a character to have a mechanical effect in game. Murder a bunch of blind orphans and even picking up a Holy Avenger falls into the category of "Best Not".