The funny thing is that some editions of the game actually tell us the criteria for good and evil in D&D and yet people continue to debate those very aspects that have already been spelled out for us in core books. I'm not talking about the subtleties of interpreting the finer points, but rather the points the designers were carefully defining. In other words, before any alignment discussion, reading the dang books ought to be required!
Different editions defined things a bit differently. AD&D was kind of annoying in how they did it, because it poorly represented, you know, people. They tended to see most creatures as good or evil, lawful or chaotic, with neutrality as this narrow ribbon in the middle for oddballs. Because of that, they defined true neutral and chaotic neutral in weird ways. (Lawful neutral got a reasonably believable description, oddly enough.)
4e was a bit different, but it was reasonably clear.
And 3e did it the best.* They painstakingly explained what the terms meant. Unlike AD&D they made the alignments make sense. Most regular people on earth would be neutral. Neutrality is a broad band of alignment, with good and evil, law and chaos being the extremes.
Here is exactly where the dividing lines between good, neutral, and evil were set in 3e:
"People who are neutral with respect to good and evil have compunctions against killing the innocent but lack the commitment to make sacrifices to protect or help others. Neutral people are committed to others by personal relationships."
If you don't have compunctions against killing the innocent, you are evil. Period. To be good you have to be willing to go out of your way to make sacrifices to protect or help others. In other words, you have to be what we would call "heroic" in our non-fantasy lingo.
Most people like to think of themselves as "good people" but the majority of decent earthlings would be "neutral" in 3e D&D.
I think any discussion of alignment should really be prefaced by which version of the game you are basing it on. I favor 3e because it makes sense and you can place actual people into an alignment, but at least discussion could come from the same basis for understanding if people were clear which edition they were going with.
Otherwise I really have no interest in alignment debates. If you're going to throw out the D&D source material, you might as well discuss it on a philosophy or religion forum rather than a D&D forum.
/rant off
* 5e is more concise and vague, but is more like 3e than any other