The Culture doesn't really dictate morality to anyone, though. Well, assuming you don't wish to be part of their group, nor wish to impose upon them/others. The major reason they are more interventionist in this fiction is that they are essentially on a total war footing.
And, I'd point out that in the Culture, your evolutionary idea is wrong: culture humans are pretty damn similar to Humans, most differences occurring to civilization wide genetic engineering. So, yeah, they're human(this isn't a random thing, either. In one Author Note section, he references it as something deliberately done by something else). More similar to Humans as we know them than those from Warhammer 40k, in all likely hood.
And, again, what makes a Human universe better? The fact that it's humans? I mean, we've had some pretty crappy human dominated civilizations/cultures on earth, so I see no reason why a Galactic Human government would necessarily be better.