>what it is to live a good life
That has to be the universal one, no? I feel like basic moral principles, at least directed towards the in group, are pretty universal. I'd argue they are more base than even humans and are shared with certain intelligent animals.
I think this is true up to a point. But you find chasmic differences as well. Polygamy is an obvious example, or lying to non-brethren (absolutely forbidden by Christianity, permitted I believe by other religions), or the caste system (unthinkable in Islam or Christianity, a base part of reality in Hinduism), etc. You find a very different attitude to wealth in many varieties of Protestantism compared to the Catholic Church. Also, as I said, whether a human being can reach a 'blessed state' by his own efforts, which I believe most eastern religions posit, and Christianity denies. Human sacrifice is an obvious example of a disagreement with very practical consequences for conduct. It was, as is well known, widely practised in parts of the Americas until the sixteenth century. Today in the west, you find people disagreeing very passionately over 'culture war' issues, which is surely a disagreement about what the good life is. And the people who say the culture wars don't matter are positing another view of the good life, different from them both. Etc.
>That has to be the universal one, no?
Not at all.
Some believe one should come to forgive their enemies. Some believe that one should purge one's enemies from life itself. Some believe that one should have no enemies.
Some believe that material wealth is fine to a point, some believe that the desire of possesing material wealth is a source of discontent, some believe that it is the source of many or all other discontents.
Some believe that wanting to improve one's station is a source of discontent.
Some are fine with removing life if it is inconvenient (like bugs) where others value the life of a bug equally to their own.
Some are okay with their friends and family members murdering foreigners actively engaged in warfare that threatens their own life and livelihood. Some are okay with their friends and family killing the friends and family of the enemy combatants. Some are okay with killing anyone just generally in the vicinity of enemy combatants. Some say literally poisoning the well is a step too far, but that we had to write that down is evidence of other's believing differently. Some believe it's okay to murder people, communities, and entire populations because they'd rather use the land for themselves, or maybe their noses are too long and their skin is indistinguishably different.
Rereading your comment, I see the "in group" disclaimer. What's the in group? Some believe it's just their family, the people on the block, the people who have lived within a certain number of miles for a certain number of years, citizens governed by a particular government, the citizens governed by a group of governments, the people living on a particular continent, people in general, humans in general, life in general, or everything. There are people to whom no human should ever murder another for any reason. You might agree with that in theory, but would you allow a conquering force that is literally raping your friends and family to simply do so unquestioned? Would you feel vindicated striking back? There are some who have decided that the morally correct thing to do is to simply quit the land and or life itself rather than to fight back, because to them murder is wrong period.
The good life is deeply personal and absolutely not universal.