Nationalism as a political ideology and nationalism as advocacy for independence of people sharing national identity that isn’t broadly recognized as official nation are two different things sharing the same name. Nationalism as political ideology is inherently evil, as it puts interests of an artifical construct above interests of people, with specific attention put to ingroup and outgroup dynamics. Separatists movements aren’t inherently evil, with the desire of liberation being usually something everyone can stand behind, but if those movements co-opt the nationalist ideology, then they may be classified as evil in my book. Or evil-er. There are no perfect victims and such.
I feel like it’s a meaningful distinction to make.
Nationalism as a function, to use your terms, doesn’t have to specifically be a result of a desire for cultural identity. It might, but when speaking of it as a function it has a narrower meaning and doesn’t have to carry additional baggage like that, though it may often imply it. When speaking of nationalism as ideology, it definitely fits my definition of evil, by putting artifical values above the wellbeing of people, which nationalism does by definition. Following that logic, nationalism as function isn’t necessarily evil (though it may be if you flavor it with racism or such, like Confederation did), while nationalism as ideology necessarily is. I’m not sure what specifically you were disagreeing with, so I hope I made my point clearer.