I was listening to a talk by Jaye McLaughlin talking about research on aggression, unfortunately it's not posted online yet, but here is the summary:
Many studies have shown that where there is more economic inequality,
there is often more violent crime. Some evolutionary psychologists
have suggested that violence may be a functional response for males in
inequitable environments, due to increased status competition. Whilst
the association between inequality and violence has been replicated
many times, the underlying mechanisms for the association have not
previously been investigated.
You can find extensive data to support the link in the work of economists Wilson & Pickett, in their books The Spirit Level & The Inner Level. It's not quite as simple as local inequality alone drives violence, but it's close.
So, engineering humans to not be violent, would involve increasing the willingness to tolerate inequality. But highly unequal societies are less functional, many talented people can't get educated or other opportunities, and there are costs and stresses for the wealthy as seen say in South Africa - the world's most unequal major country as judged by Gini coefficient. It has been argued Rome didn't have the capacity for an industrial revolution because of slavery, the extreme in inequality (Britain never had slavery in the British Isles, because of habeus corpus).
Another angle to look at is The Needham Question: why given that gunpowder, magnetic compasses and canal locks, & paper were found earliest in China, did the Modern Age not begin there? The strongest answer is, geography. China's large rivers formed the precursor kingdoms, and China was unified exactly as and when they were joined by canals. The Treasure Ships fleet in the 1400s which might have represented the first colonial power, were able to be finished by political decree, because of that unity.
Whereas in Europe, the Romans even at the height of their power couldn't hold all the forests of Germania, where the Goths emerged to sack Rome from. Far fewer large river deltas, many impenetrable hinterlands. During the era of colonialism, the Pope's fiat to give half the New World to Portugal half to Spain, was challenged by the protestant countries the Spanish Armada & many wars couldn't vanquish. The many small competitor countries, forced changes like legalising usury, banned in the Bible, because access to finance was too critical to winning wars. Many times bigger militaries lost to new tactics & technologies.
We now live in an era of nuclear weapon proliferation. There can be no doubt we need to curb all-out wars, they are an existential threat. Elections are a way to manage the tensions that inevitably arise within countries, that otherwise lead to civil wars. Sanctions are an increasingly powerful tool in an our interconnected world. And Nato is a back-stop, for mutually-assured-destruction, & threat against any nuclear first strike.
I would say then, if you keep a lid on inequality like the Romans did, you make a pressure-cooker that can explode unpredictably, in the face of challenges. The functional aspect of violent behaviour, is challenging that inequality. And the success of groups that innovate in conflicts, has driven a huge amount of human progress. This I think makes the case that we need to manage rather than eliminate conflict and violence, or societies risk becoming unstable in the face of change and challenges.
Game theory, social contracts & the free-rider problem here: Is the tyrannicide perpetrated by William Tell morally legitimate?
The collapse of complex civilisations discussed here: How and who rebuilds the state after a revolution?
Uploading minds could end some drivers of violence for the digital minds, discussed here: Can minds be uploaded in computers?