Tested with 50 Fin, 10 Warfare, 11 Aerothurge and 10 points in 2 handed:
No Death Wish:
Death Wish:
Expected Result for additive bonus: 1256*(1(base)+2(Fin)+0.5(2handed)+1(Death Wish)=4.5)/(1(base)+2(Fin)+0.5(2handed)=3.5) = 1614.85714286
Actual Result:
1614-1692.
That seems additive to me.
I play on the steam version, with the game fully up to date.
*edit* Upon Further testing it appears Death Wish is additive with attributes when your skill in question has any relation to weapon damage and is multiplicative when it does not. So for any melee builds or bow builds, Death Wish is Additive and for spellcasters it is not. Will update my post.