D&D alignment is cosmic

So, I saw that the publishers of Pathfinder offered a rules clarification recently. If you're Good aligned, and cast an Evil-aligned spell (such as Animate Dead) twice in a row, you become Neutral. If you're Neutral and cast an Evil-aligned spell three times in a row, you become Evil.

This is, suffice to say, rather stupid.

But this raises the question 'what does Good and Evil mean' in that kind of game? You know, the ones with the 3x3 alignment grids. Because it strikes me that being 'Good' has very little to do with being, you know, a decent human being. And it's very possible to be 'Evil' and be the kind of person I'd happily associate with.

Let's look at Good here. A Good character can - and indeed is expected by the game - to be highly violent. Even murderous. Those orcs over there are objectively evil (you can tell, because magic says so). So what do you do? Slaughter them all and take their stuff. Having the 'evil' tag (or more accurately, the 'monster' tag) means that they're acceptable targets, and the game expects that you will invade their homes, slaughter all of them, loot their possessions. Sure, maybe you don't kill their children, and instead deposit them in an orphanage. Maybe you don't murder the babies is not the pinnacle of morality. Hell, a truly pacifist group of PCs will merely mind-control them into behaving how they want; normally to the monsters' detriment. How nice.
Bear in mind that this is a world where the spell Atonement exists.

Now, let's look at Evil. Evil is really easy in these worlds. Cast too many of the wrong spells, and BOOM you're now objectively Evil. It doesn't matter the circumstances, or what you achieve with that magic, you're now Evil. In fact, you could cast Animate Dead a few times, and become Evil, and stay that way, whilst being a much nicer and more morally upright individual than the orc-slaughterers above.
And then we get to the nitty-gritty of magic. Why is mind-control not evil? Why is it evil to animate a corpse (by putting some extra-planar spirit in it, apparently) whilst doing the exact same thing to make a golem is OK? Honestly, because the rulebook (and thus the game world's laws of physics) say so.

So how do we resolve this?

Good and Evil are, in these settings, objective things. They're measurable forces that have a concrete, real existance. Hell, in the outer planes you have whole worlds made of absolute, solidified Good and Evil. So, really, you just need to think of them as being forces of the world.
Alignment just means which side of the big cosmic battle you're on. It has nothing to do with your personal morality, or anything like that. Casting a spell with the 'evil' tag is no more morally wrong than casting one with the 'fire' tag might be.
Being Evil doesn't mean you're the bad guys, necessarily.