I'm ok with the idea of absolute morality in D&D. If the LG description says that putting milk in tea in Tuesdays in July is the epitome of Evil, and everybody doing so must be killed on sight, that defines what being LG entails within this narrative. The specific details need not be the same for every game. One campaign could follow Gygax interpretation of LG, another could postulate that LG is forbidden from intentional killing under any circumstances, including self defence. Both are fine
The issue of what it means for things to be "objectively evil" is a fairly complicated one in metaphysics and philosophy of language. I'm going to try and keep out of it in this thread!What does it even mean for things like this to be "objectively evil?" Reasonable people in the setting would still oppose killing tea drinkers for their milk use or the Gygaxian warcrime paladin. Would this make them evil too? What's the point?
But the idea that putting milk in tea in Tuesday in July might be the epitome of evil seems ridiculous to me. Evil isn't an arbitrary notion, an empty vessel into which anything can be poured. Good is connected to values, to human interests (and rights, if such things exist), to the wellbeing of others. Evil is the opposite of these things - indifference to value and the interests of others, in pursuit of self-interest.
It has nothing to do with putting milk in tea as such. For whatever reason, there might be a taboo against doing such a thing. But if it's evil to do that, that would be because of a promise or an obligation imposed by someone else - maybe a god. The god, in turn, presumably would have a reason for imposing such an obligation. The wrongdoing wouldn't be the putting of the milk in the tea, but the breaking of the promise or the disobedience to authority.