I'm going to take care to avoid the 20-year rule and I am an amateur historian not from the US (or China). We do however look down on killing and stealing.

Kindness

The US isn't the only modern society that thinks murder and stealing are bad but nor is "do not murder or steal" a modern concept. My era of interest was thousands of years ago (190-284 CE) on an entirely different continent (China), a brutal civil war (with famine and epidemics not helping) as millions upon millions died but that didn't stop murder investigations or that stealing being considered bad. Or that administrators that could, via their moral example, stop stealing were admired.

It was a time of conquerors and warriors, some of whom were murderers (not usually written with approval), but also those that sought to end famine, poets, inventors, administrators, philosophers, historians. There were those who sought to develop a better understanding of nature, humanity, medicine, people who sought to find a way to bring around better times.

It isn't odd that people, even in times of brutality, sought to be kind, to better the world they were in or believed laws should apply, that some restraint on abuse of power was required, that standards still existed. They weren't (usually) unrealistic, that sometimes plans could not be implemented in times of chaos, that blind eyes towards the actions of some for the need to unify. But it was not a blank check for violence just because it was a time of war or "well peace has gone, morality and kindness now need to go out of the window.".

Values back then did differ from ours, there have been thousands of years of events and influences that were not in the experience of those of ye olde era. What they deemed justice or kindness might not always look good now and didn't always cover everybody. But they still held values of what they believed to be good governance, of what would lead to prosperity and restoration of not just peace but good law. Why should modern society, why should the US, not have values that also include kindness and not committing murder?

Now for armies, yes they pillaged and looted. Armies need to get supplies, soldiers want rewards and it could send a political signal you raiding in the land of a rival. However one of the most fundamental reasons why Wei became the largest power is the agricultural colonies of Ren Jun and Zao Zhi, providing regular and reliable supplies while powerful rivals soldiers were having to live off the land. While for others it was still a necessity to raid to keep their armies intact, those deemed to engage in excessive pillaging risked consequences at the time and from those who would write about them. This didn't mean stealing was seen as a good thing to do in civilian life.

Yes, armies, by the nature of warfare, do kill. They did so back then and they do so now but often a lot of work was done to justify the act, to paint the war as necessity or virtue. Proclamations, memorials, discussions recorded aimed towards both their contemporaries and to history explaining why it was entirely necessary to raise troops and to fight. Acts of excess in wars would not go without potential repercussions and comment and it didn't mean murder in civilian life wouldn't have consequences.

Historians look at the moments of... grand glory but also the smaller moments like the little jokes, silliness, friendship. Historians look at some very dark moments in history and have to pierce through the "romanticism" that tries to make things seem not so bad. Holding things like "the Lost Cause" to the sunlight and exposing it for what it truly is. History also show the human side of civilizations and people long since passed, including acts of kindness and care, how they tried to help, how they understood the world.

Conquerors

Some of the most famous might be a better phrase for such figures. Is conquering, though involving impressive skills, more impressive than ending famine via agricultural policy, being an inventor, engineering works that helped water a city, writings that influenced people for a thousand years? While recognizing that there were plenty of people, via background, gender and timing never got such chances.

It isn't like conquerors have been hidden, there are a lot of history books on such figures, there are plays, novels, films, TV shows, video games. People are aware of conquerors in ways they perhaps are unaware of those around such conquerors or for long spells of peace. It is also unlikely people have forgotten war exists with the news, history and all the forms of entertainment that cover war.

What can be a problem in terms of history, of understanding the past, is (often revolving around conquerors) is out of date "the great man of history" theory that warps the history of that time around one mighty conqueror. Two recent threads on the subject with the latter involving /u/DanKengsington, /u/jelvinjs7, /u/kugelfang52

That this figure was born and by sheer force of will and super-human talent, single-handily shaped the era they were in, striding the lands like a colossus, conquering all before them. Makes for a fun story but utterly unhelpful in terms of understanding them or what happened. Or in learning for the future.

The "great man" is shaped by the society they were in, the opportunities that their time and fortune gave them, by those around them. No man is an island, isolated from what is going on around them and no man can shape the world alone. To understand the person, to understand their conquests and how things happened, it requires looking deeper than the figure but looking in those the conqueror fought, resources they and everyone else had, why the wars happened, society, politics, the platform they were able to build from, to the supporters whose aid they required.

In terms of great in terms of talent, there were great military men and such figures might also have talents elsewhere. There are other great figures but ones whose talents don't so much grab people's immediate attention, if we focus too much on the conqueror then we do a disservice to everyone else.

In terms of morality, great in terms of talent (and position and fortune), great in terms of morality isn't one and the same. We don't have to follow the moral example of a thousand-year dead man or ignore that sometimes such figures were not considered great examples of morality in their own time.

The Future/The Lie

If it helps, I'm unaware of a nation being conquered by "being a nation of wimps" so if the US becomes possibly the first nation to fall due to that, rather than the fundamentals that can destroy a country, at least future historians will have a fascinating case study.

Natural disasters and/or pandemics shaking legitimacy of government and weakening the defenders, resource gaps, weak central authority with less grip on the levers of power and resources than might appear in theory, broken tax systems, facing something new, internal infighting, other factions going for "the enemy of my enemy is my friend", vacuums of power, bad timing and many other things yes. Nation of wimps, no.

Going "well conquerors are constant" doesn't tell us why history is full of such figures or why there are also spells of peace, why some countries avoid being invaded for a long time. Learning why such figures rose and did the things they did tell us so much more, learning why times of peace and stability ended and why times of war began. If we understand that, why not address the conditions that lead to war and conquest as more of a matter of urgency? I listed just some of the things that lead to conquest and chaos, which one might be fixed by "murder and stealing is alright"? Which fixes for the problems listed requires a lack of kindness or justice to be shown?

Does teaching them kindness and to value justice harm a chance for a nation to defend itself? I can't think of an example where that has happened but maybe others can. I can think of an example (Latter Han) where a rise in vengeance killings (and a bigoted massacre) really helped in the slide to the end of peace and to a brutal civil war that led to a sharp population decline, weakened central authority for the victor and eventually, conquest from abroad.

Teaching people what actually happened in the past, the good and the bad, may help people understand their world better and to face the challenges of their time but I don't see how teaching them to be fine with murder is going to help if they face a conqueror.

Is it a lie to teach them kindness and that stealing and killing, as others have been against for thousands of year, is wrong? No. It would be a lie to hide that history had conquerors, acts of violence or the failings of their nation. But it also not a lie to remind people of acts of kindness and generosity, it would be a lie to pretend those didn't exist.