That would be a nice addition. As it stands, I've noticed with Amurites that not everybody gives me flak for my Death Mana Soul Forge (that I sacrifice hidden nationality Tigers to, courtesy of my open borders trigger-happy neighbours), but it would be much better if some civs saw Death Mana as a bonus.
In another post it was mentioned that the Good Civs tend to get along better and, thus, traded their technologies more freely amongst one another. The Death/Entropy mana bonus between Evil Civs might not be enough to let them get along, but it would certainly help keep them from fighting one another constantly and maybe turn their attentions to the good guys who, by then, would be -8 furious! Can we say 'Bannor Crusade'?
The only trouble, I think, would be the flavour to the Amurite civilization. Evil civilizations are supposed to have trouble with Amurites for regulating Death and Entropy magics, in particular the Sheaim. Perhaps an equal modifier for the Amurites that would pop up with the Death Mana bonus that would effectively nullify it:
+4 You use Death Magic,

-4 You place sanctions on your necromancers! >:O
+4 You use Entropy Magic,

-4 You place sanctions on your uhm... entropic mages... >_>
or a simple -4 you regulate Death Magic,
But yes, need much more Good vs. Evil slaughterhousing.