What if the structure of the universe says to do something horrible?

If the "structure of universe" is something mathematical (e.g. the prime number theorem) then it's meaningless to ask "what if the structure of the universe says X" unless it truly says X. Assuming it says something different from what it really says immediately leads to a logical contradiction which allows deducing anything at all

If you could write the stone tablet yourself, what would it say?

You're suggesting that we should trust our moral intuition instead of looking for a fundamental moral principle. But my moral intuition is telling me to look for a fundamental moral principle. Apparently I'm the only one with this kind of intuition, judging by

and I hear from quite a lot of people who are happy to rhapsodize on how Their-Favorite-Morality is built into the very fabric of the universe