Whew,
seems my post has inspired quite a discussion. Actually, the cause of my original post wasn't so much the question if the dark gods are evil (in a moral sense) but if they are in the peculiar magical sense of the Master Atlas (the Subject Title wasn't very well-formulated). It was clear to me that they were of the Essaence, but it was unclear whether they used the Unlife as a power source or if they were Essaence Users - and to what effect for their followers. The same holds for Essaence-Users summoning Demons of Essaence - are their spells fed by the Unlife or by something else. The main practical reason for the post was if the Gods, and their followers more so, are immune from the influence of the Unlife and can maintain autonomy.
Personally, I like Lynn's approach with two different evil powers ("Unlife" and "Dark Essaence") very much, since it gives some of the evil guys more independence, can explain their constant discord and gives opportunity for some strange coalitions: "In face of the Unlife, these followers of Andaras don't seem so bad at all, do they?"
It creates some new questions, however. First would be, do both powers really detect as evil? With a new evil power, it would be equally plausible that the Unlife couldn't be detected at all by an Essaence spell.
Second question is how the GM can draw a clear (for the PCs) demarcation line between Unlife and Dark Essaence concerning spells, spell lists and character classes? One solution would be to rule that certain spells and spell lists can only be learned by followers of the Unlife or the Dark Essaence. Good examples for unlife-only-spells are in my opinion "Dark Absolution" and "Call of the Void". I think no essaence-user, how "dark" he might be, would ever use such a spell. Some Spell Lists, especially the demon summoning and mastery lists could be simply doubled up - if you learn one, you can only summon/master demons of essaence or demons of the void.
But a lot of the evil spells aren't that clear. Like Nejira wrote, the necromancer could probably get power from both sources.
Third question is whether one wants to introduce a corrupting effect of the Dark Essaence similar to that of the Unlife, with the notable difference that the character is transforming more and more into an immoral sadist rather than a mad destroyer?
Think about it.