The thing in DF is that everyone can be a shade of good/neutral/evil because they're not defined as such.
There's no [GOOD] token in creature_standard for the playable species (human, dwarf, elf), only some creatures have one (unicorn, fairy, etc..) .
But there are creatures for which it's not true, like Goblins are the one with an alignment token, and it's [EVIL] , like Trolls, Ogres, Nightwings etc.. and so shouldn't be able to be good/neutral/shade of them.
If Toady wanted to go away with creatures that are true evil in their nature, he would not have included that [EVIL] token in the creature definition, only in the entities (so only specific civs , not specific individuals would be true evil)
But the danger of doing that is that unless Toady seriously improve a lot how you recognize people and their factions and the reactions of them toward you (instead of current "everyone is mostly passive to you" that i think is a real bug instead of a design choice considering how uneventful it makes the game half of the time), it's only going to make adventure mode even more of a mess than it is in df2014.
Already in current version, despite the existence of [EVIL] for goblins, you can have an army made of goblins and humans fighting a human town made of goblins and humans belonging to the same civ as you, while... none on both side (even the army not belonging to your own civ) being hostile to you.