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.
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.
Having made friends of enemies, I may or may not depending on the persons development in future, and that's my point. There are many people who would agree that just because someone had done something bad in the past does not warrant continued future negative engagement. But there are also people who will make a lifelong arch-nemesis when getting cut off in traffic.
In the same village model, you'll see villages who will excommunicate people over relatively minor things, some only in severe cases, and some who will offer paths to redemption in all but the most severe of cases. A church might give you a life time ban for skipping a meeting, wearing the wrong clothing, talking to the wrong people, or any of a list of what an average person might call small transgressions.
Homophobes will taunt and even shun their friends if their friends talk to queers, god forbid befriend a fag. Christians will admonish those who talk to declared Satanists, those who listen to satanic music, etc. Some christains are homophobic, some are not. Some Satanists are homophobic, some are not. Some Christains don't mind homophobia, some can't tolerate it. So what's the core good life shared between all of these match-ups? Does the one universal good life include gay people and communication with gay people? How is that there can be these groups of people who are happy to hate on gay people if the one universal good life does not include bigotry? And you can't say that the one universal good life includes bigotry because there are many people who cannot abide such a stance toward anyone.
Even if you abstract all the way to "all in groups are wary of their outgroups" you're still not saying anything entirely true because their are in fact groups of people who believe that all are genuinely equal in all things. Do I personally? No. I believe that there are bad people and that there are people that humanity would be better off without. But, I also know that there are people who do not believe that there are bad people, only bad actions - that everyone is elligible for equality at some point.
I believe that the bible teaches the purpose of life is to glorify the God of the universe. If a religion's "purpose of life" is about "interacting with other humans, and not about God" then we have opposing "purposes". I saw something similar about Secular Humanists: they claimed they align with the bible "Love God and love your Neighbor"... but they don't believe God exists, so they - on a fundamental level - can't follow thay commandment.
All window dressing imo over the real morals at play under all these layers that society puts on top and frequently changes by the decade. What do liberals and conservatives want out of their countrymen for example? The same exact thing ultimately: good people who might lend them a cup of sugar and won't club them over the head for one. Of course propaganda has blinded people on either side into vilifying what is at the end of the day a fungible human with the same basic needs and wants as most other social animals. At the most base we have the moral principles a stray dog seeks out in either fellow dogs or a sympathetic human: let me find someone who won't fight me off and may in fact share things and help me out while I do the same for them.
The historical prominence of the slave trade in both the Christian and Islamic worlds should make it quite clear that it was hardly "unthinkable" but instead very rationalizable.
Rigid systems of social power hierarchy and oppression based on ancestry, perceived race, and occupation (which is a lot of what the worst part of caste is) were very much features of Christian and Islamic societies, even if their religions didn't codify them.
The "Hindu" caste system results from the history of social hierarchy in the Indian subcontinent. What you are calling "Hinduism" is a relatively recent construct that has emerged as a response to religions with more centralized/uniform doctrines and institutions. There are belief systems categorized under this "Hinduism" label that overtly reject caste systems (and have done so for centuries), while others overtly uphold it.
Other societies also have caste systems, just differently rationalized than the one present in some belief systems that fall under the "Hindu" banner. I'm not excusing any of these systems whether Hindu or not - they are all a blight on humanity - but they are also a feature of many societies whether or not they use the particular term "caste".
Some clear examples of excluded/oppressed castes outside of the South Asian context are the Burakumin in Japan, the al-Akhdam in Yemen, and the Mestizos with more indigenous ancestry in Latin America.
The existence of caste systems outside Hinduism doesn't change this either.
Some objects are red in and of themselves, while others can be made red by something outside them. Likewise, some beliefs cause people to act in certain ways, while other people may act in the same ways tangentially to, or even despite, their belief.