"You give" implies the ability to decide that. The problem is that there's always the threat of people such as slavers, religious fanatics, ethnonationalists etc coming to power. The more that power is centralized, the more they can do with it.
And yes, conversely, when power is decentralized, you don't get "benevolent dictators" who might otherwise do some good. My point is that this argument doesn't really work if applied consistently in all cases - or, at least, not without more digging into the specifics of costs vs benefits. Personally, I'm not convinced that centralization is a net good overall, if only because it makes large standing armies possible.
the civil war was about slavery
If anything the federal government was too accommodating and it still wasn't enough for the slave masters who dominated southern politics
Sure it was about slavery as the wedge issue over which to decide whether you can leave the union. This sort of reactionary rhetoric just skirts over the surface.
It's not like the confederacy was fighting for slavery and to be part of the union. Both Kentucky and Delaware were happy with the status quo. The confederacy wanted both slavery and to leave the union, which they perceived to be their right.
The federal government at the time was much smaller than the current one and much less empowered.
The union was good for them as long as union was supporting the slavery. They were perfectly ok with forcing free states to return escaped slaves. They were perfectly ok with the system in which north blacks could be kidnaped to slavery and had little legal recours.
The union stopped being good enough for them, when it stopped being perfect.
Speaking as a foreigner who immigrated to the US, I don't see Americans as a single nation. Maybe that was the case at some point in the past, but there are too many cultural divisions by now that transcend compatriotism.
In the past, if you traveled from San Francisco to new Orleans you would experience an entirely different set of stores, a new dialect, new customs, and frankly a new language.
Even the difference between California and Oregon is stark. California being mainly Hispanic and Oregon being mainly Nordic and with lots of Slavic influence. The names change and even the languages do.
I'd never see Russian in California but it's a regular occurrence in Oregon.
EDIT: By no means am I suggesting though that Americans do not have fondness for one another. I just think the relationship between a rural Alabaman and an urban Californian is more like the relationship between two citizens of differing EU countries. We certainly have a shared culture of sorts, and we will defend each other in the face of non-Americans, but there are cultural differences that cannot be simply glossed over.