interestingly nobody references a thematic work of fiction; named Lord of the Rings; that most of us are intimately familiar with. Here Prof. Tolkien shines a light on the true motive behind all evil: Greed. All the bad things that can be done are a function of Desire. Looking at this, and not seeing a flaw in this thinking, I think evil CAN be quantified. The ultimate desire: owning the world and doing with it what you want is just the final thing any ultimate villain can desire.
The opposite is true as well: if greed = Evil, then giving = Good, and this holds true as well: Paladins sacrificing life and limb to keep the demons from entering this world, Sages spending decades studying a disease or curse to lift it, and player characters working together to finish the mission.
That is why evil parties cannot exist and why evil is bound to lose... unless in real life
On the issue of evil magic: It is not like there is good power and bad power, it is what is done with it that makes it good or evil. Just as with electricity it doesn't matter if it is negative or positive (yeah sorry) because it is an endless chain that is supplied from either end... Whether you plug in your blender, or your cattle prod torture device, the power behind it is the same.
This said: I kinda like iconic games, where the PCs might have a problem defining evil at first, but when it is lit up, they can go in to destroy it whichever way they can. I know my players like it that way and it makes for excellent gaming, but I can see why other people like to see a whole lot of grey area, because that is how it is in real life. Big companies are not all Sauronic entities bent on corruption and total destruction, they just want to make money, and this kind of greed is fine, as long as they are giving back in the process; a product that is worth it.
Funny that the last jokers were in a way right (You twinky and toilet roll bandits!)